Implement mg_http_parse_header2()

Which is a replacement of (deprecated) `mg_http_parse_header`, but,
similarly to `asprintf`, allocates a new buffer if the client-provided
one is not large enough.

Also use it throughout mongoose code, and thus some header-related
limitations are removed; in particular,
https://github.com/cesanta/mongoose/issues/813 is fixed.

CL: Mongoose Web Server: Deprecate `mg_http_parse_header()` and implement `mg_http_parse_header2()` instead, which allocates a new buffer if the client-provided one is not large enough (similarly to `asprintf`).
CL: Mongoose Web Server: Fix limitations of header value lengths, e.g. when parsing authentication headers such as nonce, etc.

PUBLISHED_FROM=c75b1bbbbdb294ea85075ce69b1368f115fdd1ef

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

@@ -10,6 +10,7 @@ items:
   - { name: mg_get_http_var.md }
   - { name: mg_http_check_digest_auth.md }
   - { name: mg_http_parse_header.md }
+  - { name: mg_http_parse_header2.md }
   - { name: mg_http_reverse_proxy.md }
   - { name: mg_http_send_error.md }
   - { name: mg_http_send_redirect.md }