pkgin updates using +BUILD_DATE and pkg_add -U

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

Hi,

For anyone who may be interested, I'm currently testing a patched
version of pkgin which resolves a couple of the biggest problems we've
had with it.

Firstly, how to handle package updates where either a recursive
PKGREVISION bump has been forgotten or not performed correctly,
resulting in e.g. a shared library change breaking dependencies that
are already installed, for example:

  https://github.com/joyent/pkgsrc/issues/80

Secondly, changing the upgrade mechanism to use 'pkg_add -U' instead
of pkg_delete followed by pkg_add, resolving many issues (e.g.
removing openssl that pkg_install depends on) and improving
performance.

I also took the opportunity to improve the output formatting a bit,
making it closer to apt-get which is what it's supposed to take its
inspiration from.

The details are in the following gist:

  https://gist.github.com/jperkin/5584bef9bebc4c3917fdfc30d9db3bd8

and an example run of me using it to update my macOS desktop is here:

  https://gist.github.com/jperkin/e66e78e7087af501a8cb545a0790920b

I'll have patches up for review sometime next week, but am curious if
anyone has any major objections to the approaches taken or suggestions
of further improvements.

Thanks,

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