Fix pkg_install on macOS Monterey

[Jonathan Perkin <jperkin%joyent.com@localhost>]

So, Apple's crappy virtual memory system strikes again, and the problem 
we're running into is that the +INSTALL and +DEINSTALL scripts executed 
during upgrade of pkg_install fail because they're using fork() when the 
original file system image of pkg_add and pkg_delete are either missing 
or changed, as we're in the process of replacing them.

At least that's the best theory I'm working on right now, given the 
inability to do any proper debugging on this OS, and goes some way to 
explain why it's an intermittent failure.

I have a diff to switch the calls of these script to use system() 
instead:

  https://gist.github.com/jperkin/6e66a8753ec8039b298cac7b57f2a7de

and have verified that this "fixes" the issue in my limited testing, but 
I'm expecting some objections to this change (I don't like it either).

To those of you who object, please come up with some alternative 
solutions.  The only ones I can think of are really really bad.

Cheers,

--
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com