Changed on-request job running to shell out instead of doing a loop.

• Also factored out a new wfShellExecDisabled() function.
• This will keep the process in the background if possible to avoid killing site performance, especially with slow jobs.
• This also keep fatals and uncatcheable exceptions from hitting the user.
• If $wgPhpCli is not set to an actual path or safe mode is on, then the old code will be used.

Change-Id: I6a28152251659ee53eee2604f16d5bf02c85a44f