Better handling of errors during the fork

Adds two additional error checks when the shell forks:
1. Checks for permission (+r, +x)
2. Checks if the file exists

The first error may occur if the user does not have read access to the
file, or if the file is not executable. The second error may occur if a
file was removed from the $PATH, and the $PATH hasn't been refreshed
yet.

0 files changed, 0 insertions, 0 deletions