RBTools has wrappers for creating temp files without worrying about

cleanup, handling that automatically during the shutdown of the process.

The existing support, however, did not allow callers to control the

naming of any temp files, and our defaults made it hard to distinguish

RBTools temp files from those of other processes.
This change adds new arguments to make_tempfile(), allowing a custom

suffix and prefix to be specified. By default, a prefix of rbtools. is

used.
It also allows an explicit filename to be specified. This will be useful

for some upcoming work on the commit message editing capabilities. When

providing an explicit filename, a temp directory will be created with

the temp file living inside of it. This avoids conflicts when multiple

callers are trying to create temp files of the same explicit name.
Unit tests have been updated for the new additional functionality, and

some existing tests have been fixed to better meet our standards.

Unit tests pass.
Made use of the new capabilities in a couple upcoming changes.