Wrappers for safely calling commands in a new PAG

Wrappers for safely calling commands in a new PAG

The usual mechanism for starting a new PAG is pagsh(1). Unfortunately, because
it basically just execvp(3) /bin/sh passing the appropriate arguments, it isn't
immediately obvious how to safely pass arguments that may contain shell
metacharacters. By using the shell's exec and taking advantage of the fact that
later arguments to /bin/sh end up in $@ we can safely avoid shell
metacharacters. We wrap subprocess.check_{call,output} in
pag_check_{call,output}, which perform appropriate contortions to establish the
PAG before safely executing the passed commands without evaluating any
metacharacters.