Minor docs fix: ./pants.d -> .pants.d

Fix path in the http://pantsbuild.github.io/tshoot.html page: ./pants.d  -> .pants.d

Testing Done:
./build-support/bin/publish_docs.sh -d tmp -o; verified the fix.

Reviewed at https://rbcommons.com/s/twitter/r/1387/
Make Pants goal run for Python exit with error code 1 if the python
program exits non-zero (as pants goal run does for java).

Testing Done:
Ran new tests.

Reviewed at https://rbcommons.com/s/twitter/r/1374/
Fix .proto files with unicode characters in the comments. (Problem found in '_same_contents')

Testing Done:
Updated unit tests, added a testproject example.

I tried naming the .proto file with a utf-8 character and it failed miserably. Fortunately I don't have that case, so I didn't try to fix it.

Bugs closed: 792

Reviewed at https://rbcommons.com/s/twitter/r/1330/
Remove execute_on_subproc and background subproc pool

Turns out we didn't get the parallelism we wanted with threads making blocking calls to subprocesses.

Re-using the subproc_map that is used in cache reads, and sending the whole call as a single piece of Work,
gets parallel compression and uploads, though at the expense of slightly less granular reporting. It also
turns out that separate process pools for aren't that useful, and slows down startup/shutdown.
Make all access to Context's lock via helpers

`context._lock` should be an implementation detail with callers only concerned with acquire_lock and release_lock.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/41820893

Bugs closed: 818

Reviewed at https://rbcommons.com/s/twitter/r/1391/
Get rid of the old config registration mechanism.

It was hardly used, and is now superceded by the new options system.

Moved a bunch of accesses of pants_workdir from config to new options.
There are still several such accesses that will take more work, as
they are in code locations with no reference to an Options instance.

To do so, made bootstrap option values available during option registration.
This uses function properties, which actually seemed like a reasonable
solution here.  Yay python for making everything an object.

Testing Done:
CI passes.

Reviewed at https://rbcommons.com/s/twitter/r/1385/