R1:80643d63a8f8

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…