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/url.c

@@ -128,7 +128,6 @@ bool curl_win32_idn_to_ascii(const char *in, char **out);
 #include "http_proxy.h"
 #include "conncache.h"
 #include "multihandle.h"
-#include "dotdot.h"
 #include "strdup.h"
 #include "setopt.h"
 #include "altsvc.h"