Detect empty $PATH environmental var

Summary:
By default, PHP-FMP (an alternate PHP FCGI SAPI) cleans the entire environment
for child processes. This means we have no $PATH.

This causes some confusing failures for reasons I don't fully understand. If you
do these things:

exec_manual('env');
exec_manual('export');

...they show no $PATH, as expected. If you do this:

exec_manual('echo $PATH');

...it shows a path. And this works (i.e., it finds the executable):

exec_manual('ls');

...but this fails (it says "no ls in ((null))"):

exec_manual('which ls');

So, basically, the sh -c process itself gets a default PATH somehow, but its
children don't. I don't realllly get why this happens, but clearly an empty
$PATH is a misconfiguration, and can easily be remedied.

See discussion here: https://github.com/facebook/libphutil/issues/7

Test Plan: Applied patch to Centos6 + nginx + PHP-FPM machine, ran setup, the
configuration issue was detected and I was given information on resolving it.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D1413