test: Port atomicity test to Python

Previously, this was implemented using a horrible GDB script (because
there is no such thing as a non-horrible GDB script).  This GDB script
often broke with newer versions of GDB for mysterious reasons.  Port
the test script to GDB's Python API, which makes the code much cleaner
and, hopefully, more stable.

(cherry picked from commit cbbda62258360f035894cff9dfd66c60b0cc707f)

Conflicts:
	test/T380-atomicity.sh
test: simplify T360-symbol-hiding, use nm instead of objdump

After yet another variation in objdump output caused this test to fail
(on a Debian port, no less), I decided whatever putative benefit we
get from looking at the object files instead of the library isn't
worth the maintenence headache.

This version uses nm -P. nm -P should be portable, and fixed format.
It purposely doesn't use the -D argument, since that is non-POSIX and
nm on GNU/Linux seems do the right thing without it.

It still won't work out of the box on e.g. Mac OS/X. I think the right
thing to do there is to move some more configuration information into
sh.config.

(cherry picked from commit c34d6bad0f9da300eac2181e2073aee130432932)
test/emacs: force *Messages* buffer to be writable

In emacs 24.4 the messages buffer starts being read-only, which kills
these tests.  This seems to be the point of the variable
inihibit-read-only, which has existed at least since emacs 21.
test: kill '"filename": "signature.asc"' from json output

This starts to appear with emacs24.4, so we can't easily have it in
our expected output.
version: bump to 0.18.2~rc1
NEWS: add minimal news item for 0.18.2

The less said, the less typos to make.
debian: add changelog stanza for 0.18.2~rc-1
debian: build depend on dh-python

The build log asked me nicely, so I did.
Merge tag '0.18.2_rc1'

notmuch 0.18.2~rc1 release

Conflicts:
	NEWS
	debian/changelog