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/mg_http_parse_header2.md

@@ -0,0 +1,30 @@
+---
+title: "mg_http_parse_header2()"
+decl_name: "mg_http_parse_header2"
+symbol_kind: "func"
+signature: |
+  int mg_http_parse_header2(struct mg_str *hdr, const char *var_name, char **buf,
+                            size_t buf_size);
+---
+
+Parses the HTTP header `hdr`. Finds variable `var_name` and stores its value
+in the buffer `*buf`, `buf_size`. If the buffer size is not enough,
+allocates a buffer of required size and writes it to `*buf`, similar to
+asprintf(). The caller should always check whether the buffer was updated,
+and free it if so.
+
+This function is supposed to parse cookies, authentication headers, etc.
+Example (error handling omitted):
+
+    char user_buf[20];
+    char *user = user_buf;
+    struct mg_str *hdr = mg_get_http_header(hm, "Authorization");
+    mg_http_parse_header2(hdr, "username", &user, sizeof(user_buf));
+    // ... do something useful with user
+    if (user != user_buf) {
+      free(user);
+    }
+
+Returns the length of the variable's value. If variable is not found, 0 is
+returned. 
+