Design for new lock extension

[timeless]

For the record...

* having <lock> and <locks> as commands sucks

i'm told this matches bookmark/bookmarks, so it'd be nice to have this
design decision documented.

* djc noted that --force instead of --steal

and I agree

* and it should be hg command [--flags] FILEs

* there needs to be an explanation of how email works
-- some of us have really bad experiences w/ email

* i'd expect hg pull to tell me if a lock i have was broken, when,
why, and by whom

* offhand i'm pretty sure i'd want some hooks for remote-repositories
1. to e.g. say <no-this-user-may-not-lock-this-file>
2. or to allow the repo to provide a custom email message when a lock is broken

* in theory you could provide something in the {append} hook which
prevents someone from pushing stuff if they don't have a lock and
they're changing it. bonus points for including a sample for that