Fix reading of the request path when running the PHP builtin webserver

Summary:
Ref T13575. Since PHP builtin webserver support was added, the pathway for parsing request parameters became more complex. We now rebuild "$_REQUEST" later, and this rebuild will destroy any mutations made to it here, so the assignment to "path" is lost.

Instead of "validating" the request path, make this method "read" the request path and store it explicitly, so it will survive any later request mutations.

Test Plan:

• Submitted any POST form while running Phabricator under the builtin PHP webserver. Old behavior was an error when accessing "path"; new behavior is a working application.
• Loaded normal pages, etc.

Maniphest Tasks: T13575

Differential Revision: https://secure.phabricator.com/D21506