tests: make individual test sources compile cleanly

Tidy up headers and includes to ensure all individual test source
compile cleanly (but not link). To allow running clang-tidy (and
possibly other static analyzers) on them. It also improves readability
and allows to verify them locally, without the bundle logic.

clang-tidy ignores #included C files, so it's blind to bundle C files
the include these tests. The current workaround of embedding has
a couple of downsides:. meaningless filenames and line numbers,
missing issues, messing up self header paths. Thus, running it on
individual sources would be beneficial.

Also:
- de-duplicate includes.
- untangle some includes.
- formatting/indentation fixes.
- merge `getpart.h` into `first.h`.

Ref: https://github.com/curl/curl/pull/17680#issuecomment-2991730158

Closes #17703

tests/server/first.h

@@ -23,6 +23,8 @@
  * SPDX-License-Identifier: curl
  *
  ***************************************************************************/
+#include "curl_setup.h"
+
 typedef int (*entry_func_t)(int, char **);
 
 struct entry_s {
@@ -30,4 +32,127 @@ struct entry_s {
   entry_func_t ptr;
 };
 
+extern const struct entry_s s_entries[];
+
+#ifndef UNDER_CE
+#include <signal.h>
+#endif
+#ifdef HAVE_NETINET_IN_H
+#include <netinet/in.h>
+#endif
+#ifdef HAVE_NETINET_IN6_H
+#include <netinet/in6.h>
+#endif
+#ifdef HAVE_ARPA_INET_H
+#include <arpa/inet.h>
+#endif
+#ifdef HAVE_NETDB_H
+#include <netdb.h>
+#endif
+
+#include <curlx.h> /* from the private lib dir */
+
+/* adjust for old MSVC */
+#if defined(_MSC_VER) && (_MSC_VER < 1900)
+#  define snprintf _snprintf
+#endif
+
+#ifdef _WIN32
+#  define CURL_STRNICMP(p1, p2, n) _strnicmp(p1, p2, n)
+#elif defined(HAVE_STRCASECMP)
+#  ifdef HAVE_STRINGS_H
+#  include <strings.h>
+#  endif
+#  define CURL_STRNICMP(p1, p2, n) strncasecmp(p1, p2, n)
+#elif defined(HAVE_STRCMPI)
+#  define CURL_STRNICMP(p1, p2, n) strncmpi(p1, p2, n)
+#elif defined(HAVE_STRICMP)
+#  define CURL_STRNICMP(p1, p2, n) strnicmp(p1, p2, n)
+#else
+#  error "missing case insensitive comparison function"
+#endif
+
+enum {
+  DOCNUMBER_NOTHING    = -7,
+  DOCNUMBER_QUIT       = -6,
+  DOCNUMBER_BADCONNECT = -5,
+  DOCNUMBER_INTERNAL   = -4,
+  DOCNUMBER_CONNECT    = -3,
+  DOCNUMBER_WERULEZ    = -2,
+  DOCNUMBER_404        = -1
+};
+
+#include <curl/curl.h> /* for curl_socket_t */
+
+#ifdef USE_UNIX_SOCKETS
+#ifdef HAVE_SYS_UN_H
+#include <sys/un.h> /* for sockaddr_un */
+#endif
+#endif /* USE_UNIX_SOCKETS */
+
+typedef union {
+  struct sockaddr      sa;
+  struct sockaddr_in   sa4;
+#ifdef USE_IPV6
+  struct sockaddr_in6  sa6;
+#endif
+#ifdef USE_UNIX_SOCKETS
+  struct sockaddr_un   sau;
+#endif
+} srvr_sockaddr_union_t;
+
+/* getpart */
+#define GPE_NO_BUFFER_SPACE -2
+#define GPE_OUT_OF_MEMORY   -1
+#define GPE_OK               0
+#define GPE_END_OF_FILE      1
+
+extern int getpart(char **outbuf, size_t *outlen,
+                   const char *main, const char *sub, FILE *stream);
+
+/* utility functions */
+extern char *data_to_hex(char *data, size_t len);
+extern void logmsg(const char *msg, ...);
+extern void loghex(unsigned char *buffer, ssize_t len);
+extern unsigned char byteval(char *value);
+extern int win32_init(void);
+extern const char *sstrerror(int err);
+extern FILE *test2fopen(long testno, const char *logdir2);
+extern curl_off_t our_getpid(void);
+extern int write_pidfile(const char *filename);
+extern int write_portfile(const char *filename, int port);
+extern void set_advisor_read_lock(const char *filename);
+extern void clear_advisor_read_lock(const char *filename);
+static volatile int got_exit_signal = 0;
+static volatile int exit_signal = 0;
+#ifdef _WIN32
+static HANDLE exit_event = NULL;
+#endif
+extern void install_signal_handlers(bool keep_sigalrm);
+extern void restore_signal_handlers(bool keep_sigalrm);
+#ifdef USE_UNIX_SOCKETS
+extern int bind_unix_socket(curl_socket_t sock, const char *unix_socket,
+                            struct sockaddr_un *sau);
+#endif
+extern unsigned short util_ultous(unsigned long ulnum);
+
+/* global variables */
+static const char *srcpath = "."; /* pointing to the test dir */
+static const char *pidname = NULL;
+static const char *portname = NULL; /* none by default */
+static const char *serverlogfile = NULL;
+static int serverlogslocked;
+static const char *configfile = NULL;
+static const char *logdir = "log";
+static char loglockfile[256];
+#ifdef USE_IPV6
+static bool use_ipv6 = FALSE;
+#endif
+static const char *ipv_inuse = "IPv4";
+static unsigned short server_port = 0;
+static const char *socket_type = "IPv4";
+static int socket_domain = AF_INET;
+
+#define SERVERLOGS_LOCKDIR "lock"  /* within logdir */
+
 #endif /* HEADER_SERVER_FIRST_H */