Move mg_a*printf to common

PUBLISHED_FROM=cccd28ef43ec4e08fa52340ce78bda10db5ac27a

docs/c-api/util.h/intro.md

@@ -3,8 +3,6 @@ title: "Utility API"
 symbol_kind: "intro"
 decl_name: "util.h"
 items:
-  - { name: mg_asprintf.md }
-  - { name: mg_avprintf.md }
   - { name: mg_base64_decode.md }
   - { name: mg_base64_encode.md }
   - { name: mg_basic_auth_header.md }

docs/c-api/util.h/mg_asprintf.md

@@ -1,22 +0,0 @@
----
-title: "mg_asprintf()"
-decl_name: "mg_asprintf"
-symbol_kind: "func"
-signature: |
-  int mg_asprintf(char **buf, size_t size, const char *fmt, ...);
----
-
-Prints message to the buffer. If the buffer is large enough to hold the
-message, it returns buffer. If buffer is to small, it allocates a large
-enough buffer on heap and returns allocated buffer.
-This is a supposed use case:
-
-   char buf[5], *p = buf;
-   mg_avprintf(&p, sizeof(buf), "%s", "hi there");
-   use_p_somehow(p);
-   if (p != buf) {
-     free(p);
-   }
-
-The purpose of this is to avoid malloc-ing if generated strings are small. 
-

docs/c-api/util.h/mg_avprintf.md

@@ -1,10 +0,0 @@
----
-title: "mg_avprintf()"
-decl_name: "mg_avprintf"
-symbol_kind: "func"
-signature: |
-  int mg_avprintf(char **buf, size_t size, const char *fmt, va_list ap);
----
-
-Same as mg_asprintf, but takes varargs list. 
-