urlapi: leaner with fewer allocs

Slightly faster with more robust code. Uses fewer and smaller mallocs.

- remove two fields from the URL handle struct
- reduce copies and allocs
- use dynbuf buffers more instead of custom malloc + copies
- uses dynbuf to build the host name in reduces serial alloc+free within
  the same function.
- move dedotdotify into urlapi.c and make it static, not strdup the input
  and optimize it by checking for . and / before using strncmp
- remove a few strlen() calls
- add Curl_dyn_setlen() that can "trim" an existing dynbuf

Closes #9408

lib/dynbuf.h

@@ -38,6 +38,7 @@
 #define Curl_dyn_len(a) curlx_dyn_len(a)
 #define Curl_dyn_reset(a) curlx_dyn_reset(a)
 #define Curl_dyn_tail(a,b) curlx_dyn_tail(a,b)
+#define Curl_dyn_setlen(a,b) curlx_dyn_setlen(a,b)
 #define curlx_dynbuf dynbuf /* for the struct name */
 #endif
 
@@ -63,6 +64,7 @@ CURLcode Curl_dyn_vaddf(struct dynbuf *s, const char *fmt, va_list ap)
   WARN_UNUSED_RESULT;
 void Curl_dyn_reset(struct dynbuf *s);
 CURLcode Curl_dyn_tail(struct dynbuf *s, size_t trail);
+CURLcode Curl_dyn_setlen(struct dynbuf *s, size_t set);
 char *Curl_dyn_ptr(const struct dynbuf *s);
 unsigned char *Curl_dyn_uptr(const struct dynbuf *s);
 size_t Curl_dyn_len(const struct dynbuf *s);