SSL: guard use of all SSL options for bug workarounds.

Some of the OpenSSL forks (read: BoringSSL) started removing unused,
no longer necessary and/or not really working bug workarounds along
with the SSL options and defines for them.

Instead of fixing nginx build after each removal, be proactive
and guard use of all SSL options for bug workarounds.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>
Core: ngx_regex_compile() error handling fixes.

Now we actually return NGX_ERROR on errors, and provide an error
string for memory allocation errors.

Reported by Markus Linnala.
Fixed ETag memory allocation error handling.

The etag->hash must be set to 0 to avoid an empty ETag header being
returned with the 500 Internal Server Error page after the memory
allocation failure.

Reported by Markus Linnala.
Added ngx_init_setproctitle() return code check.

The ngx_init_setproctitle() function, as used on systems without
setproctitle(3), may fail due to memory allocation errors, and
therefore its return code needs to be checked.

Reported by Markus Linnala.
Upstream: avoided directly terminating the connection.

When memory allocation failed in ngx_http_upstream_cache(), the connection
would be terminated directly in ngx_http_upstream_init_request().
Return a INTERNAL_SERVER_ERROR response instead.