common: Run spawned commands from $PATH

This makes us less fragile, when commands move between
directories.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
shell: Use $PATH when executing various spawned commands

This will simplify porting efforts, between OS releases
and distros.

Closes #887
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>