task: Capture both stdout and stderr into the same file

It's handy because we watch the same file in order to see what is
happening.
builddisks: List of allowed tree suffixes

Define a list of tree suffixes that will be used to generate disk images.
builddisks keeps generating -runtime images.
zdisks: Build compressed disk images for -devel too

This is a much more convenient for people who know they want
developer tools.

Closes #29