Rather than cram most .gitignore content into the top-level .gitignore
file, partition it into lower-level .gitignore files, as is already done
with a couple subdirectories, just to be consistent.
I'm sure these files could be even tidier but this seems sufficient for
a first pass.
Change-Id: I0cc8cb2a9d7fa82c4f6de1aef9f82107c8cc7a6a
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
