Make skydb just a ctl-style helper (e.g. apachectl)

Make skydb just a ctl-style helper (e.g. apachectl)

This makes it much easier to work with a device where we
don't expect a persistent commandline session.  Prompt no longer
actually makes a prompt but rather just runs an http server.

This is just a v1 patch.  It doesn't work with android yet
and it isn't smart enough to allow you to run more than one
copy of prompt.cc at the same time (since you can't control
the port it listens on).  If you have a second copy of
prompt runnning (or anything else bound to port 7777) it will
just crash.

We could make this a lot better, including splitting out the
pid-file tracking for sky_server into sky_server instead of
having skydb manage it.

R=ojan@chromium.org, abarth@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/5d6b0da4dad573fb9319911f5a0535181b7df7b6

[ojan, 2015-01-07 23:49:53]

ojan@chromium.org changed reviewers:
+ ojan@chromium.org

[ojan, 2015-01-07 23:49:54]

lgtm. I didn't do a terribly thorough review, but this all seems fine.

https://codereview.chromium.org/840973002/diff/1/sky/tools/skydb
File sky/tools/skydb (right):

https://codereview.chromium.org/840973002/diff/1/sky/tools/skydb#newcode157
sky/tools/skydb:157: help='launch a new mojo_shell with sky')
This help is wrong?

[eseidel, 2015-01-08 21:01:20]

Committed patchset #3 (id:40001) manually as
5d6b0da4dad573fb9319911f5a0535181b7df7b6 (presubmit successful).