[RFC] deprecate @exec and @unexec in plists in favor of pre-install and post-install scripts

[Garrett Cooper]

Hi,
    As part of taking a look at the differences in our implementation
of pkg_install(1) in order to afford an improvement over the existing
code, I've looked at various implementations of pkg_install, one being
NetBSD's evolution [1]. It's several years ahead from our's and while
I don't believe that all of the complexity is desired, there's a lot
of good lessons to be learned from this. One of which is that they
replaced the @exec and @unexec calls with string pre-install //
post-install and pre-deinstall // post-deinstall scripts. I think that
this potentially is a good step forward because it takes some of the
guts out of the +CONTENTS files and places it in [bourne shell]
scripts, which are easier to maintain and potentially understand.
    I realize that some of the loss would be that one couldn't simply
specify things like %f, %D, %F, etc with @exec and @unexec, but that
seems a small price to pay for tuning everything a bit more. On the
plus side too, that means that one could use an extensive set of
shell, etc libraries that would avoid code duplication like what's
present in the +CONTENTS files. This is one of the small observations
I made after starting on work which would modify 1k python ports to
not install the byte-compiled or optimized files (side topic that we
can talk about in another thread if desired).
Thoughts?
-Garrett