The ASAN build fails with
```
cookie_auth.c:62:7: error: variable 'ret' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
  if (cookie_header == NULL) goto clean;
      ^~~~~~~~~~~~~~~~~~~~~
cookie_auth.c:84:10: note: uninitialized use occurs here
  return ret;
         ^~~
cookie_auth.c:62:3: note: remove the 'if' if its condition is always false
  if (cookie_header == NULL) goto clean;
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cookie_auth.c:65:3: note: variable 'ret' is declared here
  struct session *ret = NULL;
  ^
cookie_auth.c:62:7: error: variable 'ssid' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
  if (cookie_header == NULL) goto clean;
      ^~~~~~~~~~~~~~~~~~~~~
cookie_auth.c:81:7: note: uninitialized use occurs here
  if (ssid != ssid_buf) {
      ^~~~
cookie_auth.c:62:3: note: remove the 'if' if its condition is always false
  if (cookie_header == NULL) goto clean;
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cookie_auth.c:64:3: note: variable 'ssid' is declared here
  char *ssid = ssid_buf;
  ^
2 errors generated.
```

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

This reverts commit 1a17e17c462bdd4e1d26d8742f8b7087273e04c2.

PUBLISHED_FROM=80028de308c9a021955d1425d2bfee8feb85f193

PUBLISHED_FROM=f45e06aeba1c00212078d539b6f74cb8bd054326

On my way, fixed a couple of cases where we had `()` in the header, and
non-empty argument list in the source file.

PUBLISHED_FROM=5519526cf84e2bbd425a726fcc112fea1a95cbf1

Use templated index page and SSI calls instead of hard-coded html

PUBLISHED_FROM=eade9dfe283952ed5465cd0ff1924f61a14dea0c

PUBLISHED_FROM=243437cf7d48b04aef3896bc4c5d4010975299a4