POST requests: remove default post handler

`post_max_size` limits the size of the traditional upload process used
by older browsers. We'd like to have that set as high as possible,
BUT...

If php receives a POST request that is less than `post_max_size` and
it comes from a modern browser (the content type is anything other
than `multipart/form-data`), php uses a default parser that is very
memory-inneficient and needs roughly `4 * $filesize` bytes of on-heap
memory for a given upload.  This effectively is a tradeoff, set it
high and modern browsers start making php run out of memory, set it
low and older browser can't upload big files.

The default parser is useless in this case, it's designed to parse
traditional field-based POST requests. When presented with a file
upload, it simply does nothing with the data but waste memory. PHP has
left the field-parsing handler in as a default even when the POST
doesn't claim to be a `application/x-www-form-urlencoded` form for
backwards compatability reasons.  We don't need this.

This simply removes the default POST handler, allowing the two
specific ones to parse the request when needed (multipart and
form-urlencoded). All other POST requests are left up to the user-land
php code to parse (JSON, modern file uploads, XML)...

Note: this has one drawback. Previously, the body for
application/x-form-urlencoded requests would be copied in memory
and available on STDIN (even though it's already been parsed into
$_POST) and now it won't. It only affects this mime-type.