Add build-many-glibcs.py bot-cycle action.

Add build-many-glibcs.py bot-cycle action.

This patch continues the process of setting up build-many-glibcs.py to
run as a bot monitoring for and reporting on build issues by adding a
bot-cycle action to the script.  When this action is used, it will run
the checkout action (re-execing itself if it was changed by that
action), then rebuild whichever of host-libraries, compilers, glibcs
should be rebuilt based on changed versions, time elapsed and state of
previous builds.  Email is sent with the results of the build (for
each build action done).

The rebuild logic is: if previous build time or versions aren't
recorded, rebuild that component.  If the script has changed, rebuild
everything.  If any relevant component version has changed, rebuild,
except for not rebuilding compilers if the time indicated in the bot
configuration has not passed since the last build of the compilers.
If one piece is rebuilt then rebuild subsequent pieces as well.

Using bot-cycle requires a configuration file bot-config.json in the
toplevel directory used by build-many-glibcs.py.  It might contain
e.g.

{
  "compilers-rebuild-delay": 604800,
  "email-from": "Example Name <user@example.org>",
  "email-server": "localhost",
  "email-subject": "GCC 6 %(action)s %(build-time)s build results",
  "email-to": "libc-testresults@sourceware.org"
}

My next intended step is adding a further action "bot" which loops
running bot-cycle then sleeping for an amount of time given in
bot-config.json.  Then I'll set up a bot using that action (building
with GCC 6 branch; a bot using GCC mainline may wait until the SH
out-of-memory issues
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78460> are fixed; I
expect the bot to mail to me until it seems ready to switch to mailing
to gcc-testresults).

	* scripts/build-many-glibcs.py: Add bot-cycle to usage message.
	Import email.mime.text, email.utils and smtplib modules.
	(Context.__init__): Initialize self.bot_config_json.
	(Context.run_builds): Handle bot-cycle action.
	(Context.load_bot_config_json): New function.
	(Context.part_build_old): Likewise.
	(Context.bot_cycle): Likewise.
	(Context.bot_build_mail): Likewise.
	(Context.bot_run_self): Likewise.
	(get_parser): Allow bot-cycle action.