install packages as needed

[mheily]

A useful feature would be to install ports/packages as needed. A manifest could specify something like:

'Packages': ['apache24'],

and launchd(8) would automatically install the package prior to launching the job.

To support multiple operating systems, the Packages key could be a hash with the key being an (operating system triplet)[https://www.gnu.org/software/autoconf/manual/autoconf-2.65/html_node/Specifying-Target-Triplets.html]. Example:

'Packages': {
  '*-*-freebsd': ['apache24'],
  '*-*-openbsd': ['apache22'],
},