Fix simplistic `if pid` tests

Add a new function s:latexmk_check_pid(pid) to check whether the given
PID is still alive and is owned by a latexmk process.

This allows
   let g:latex_latexmk_options = '-pvc-'
i.e. compile only when explicitely called.
Merge pull request #50 from 1uc/master

Better latexmk PID check (for linux)