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

src/infrastructure/setup/PhabricatorSetup.php

@@ -122,6 +122,26 @@ class PhabricatorSetup {
         "consequences of leaving it unconfigured.\n");
     }
 
+    $path = getenv('PATH');
+    if (empty($path)) {
+      self::writeFailure();
+      self::write(
+        "Setup failure! The environmental \$PATH variable is empty. ".
+        "Phabricator needs to execute system commands like 'svn', 'git', ".
+        "'hg', and 'diff'. Set up your webserver so that it passes a valid ".
+        "\$PATH to the PHP process.\n\n");
+      if (php_sapi_name() == 'fpm-fcgi') {
+        self::write(
+          "You're running php-fpm, so the easiest way to do this is to add ".
+          "this line to your php-fpm.conf:\n\n".
+          "  env[PATH] = /usr/local/bin:/usr/bin:/bin\n\n".
+          "Then restart php-fpm.\n");
+      }
+      return;
+    } else {
+      self::write(" okay  \$PATH is nonempty.\n");
+    }
+
     self::write("[OKAY] Core configuration OKAY.\n");
 
     self::writeHeader("REQUIRED PHP EXTENSIONS");