Make statSyscall in ext_file.cpp support relative paths by default.

This fixes a number of Zend incompatibilities -- specifically this
fixes #1733, and gives relative path support that should exist to
is_readable, is_writable, is_executable, fileperms, fileowner, etc.
(all the file* functions for extracting stat fields).

Also, fix support for stream URIs for ext_file functions that rely on
statSyscall, as discussed on #1732.