[arch-projects] [devtools] Minor fix, and cleanups from Parabola

[lukeshu at lukeshu.com]

Hi,

Here's another batch of changes backported from Parabola's version of
devtools.  These changes should be pretty uncontroversial.

This patchset applies cleanly if applied over the patchsets I've
already submitted.  It shouldn't be any real trouble to apply them
without applying the previous patches, but the context of the diffs
has changed enough that it would require resolution of "conflicts".
Let me know if you want me to resubmit them with a different base.

The first 3 commits deal with strings the user sees; most
significantly: fix a bug where makechrootpkg's usage text could be
tricked into lying about default values.

The next 3 are general code quality improvements to makechrootpkg;
they shouldn't actually change anything.

The last 2 let common.sh be used in new ways... even if it isn't used
those ways in devtools.  Let it be included multiple times (like a C
header), and let it be used in scripts that `set -u`.

-- 
Happy hacking,
~ Luke Shumaker