Merge: No abort() on bad execvp

When a bad command is passed to Process, the child-process will `C-abort()`, displaying frightening messages on the screen and causing confusion to the user.
Instead, just `_exit(127)` to terminate the child silently and let the parent do its things.

TODO better error management to not lose the error of the failed `execvp` system call.

Pull-Request: #1074
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>