QFileSystemEngineUnix: Don't stat before retrieving working path.

This is entirely unnecessary. If the path is bad, then getcwd and friends will
fail. Doing an extra stat imposes an extra performance overhead without reason.

Trivia: A dive into Qt's history shows that the stat dates back to:
  Sat Aug 12 14:24:36 1995 +0100
The original purpose of the stat was to avoid calling getcwd unless the path had
actually changed. Subsequently, the caching was removed, but the stat remained.

Change-Id: Ia4598dc74ded36516b3e10e7ab0eb5a6a5690466
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Don't attempt to load an empty generic plugin if no environment variable is set.

Also warn if a requested generic plugin is not available.

Change-Id: Iab1f1f87be435b4ad446881cba25d0b19a564f08
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>