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

test/unit_test.c

@@ -4941,12 +4941,34 @@ static const char *test_http_parse_header(void) {
       "xx=1 kl yy, ert=234 kl=123, "
       "uri=\"/?naii=x,y\";ii=\"12\\\"34\" zz='aa bb',tt=2,gf=\"xx d=1234");
   char buf[20];
+  char *buf2;
 
   ASSERT_EQ(mg_http_parse_header(&h, "ert", buf, sizeof(buf)), 3);
   ASSERT_STREQ(buf, "234");
+
   ASSERT_EQ(mg_http_parse_header(&h, "ert", buf, 2), 0);
   ASSERT_EQ(mg_http_parse_header(&h, "ert", buf, 3), 0);
   ASSERT_EQ(mg_http_parse_header(&h, "ert", buf, 4), 3);
+
+  buf2 = buf;
+  ASSERT_EQ(mg_http_parse_header2(&h, "ert", &buf2, 2), 3);
+  ASSERT(buf2 != buf);
+  free(buf2);
+
+  buf2 = buf;
+  ASSERT_EQ(mg_http_parse_header2(&h, "ert", &buf2, 3), 3);
+  ASSERT(buf2 != buf);
+  free(buf2);
+
+  buf2 = buf;
+  ASSERT_EQ(mg_http_parse_header2(&h, "ert", &buf2, 4), 3);
+  ASSERT(buf2 == buf);
+
+  buf2 = NULL;
+  ASSERT_EQ(mg_http_parse_header2(&h, "ert", &buf2, 0), 3);
+  ASSERT_STREQ(buf2, "234");
+  free(buf2);
+
   ASSERT_EQ(mg_http_parse_header(&h, "gf", buf, sizeof(buf)), 0);
   ASSERT_EQ(mg_http_parse_header(&h, "zz", buf, sizeof(buf)), 5);
   ASSERT_STREQ(buf, "aa bb");