SVN traps SIGINT and attempts to clean itself up, but this results in hangs
waiting for TCP. This patch does two things: daemonizes worker threads so they
are culled when the main thread dies (is ctrl-C'd) and keeps track of spawned
subprocesses to kill any remaining ones when the main program is ctrl-C'd.

A user ctrl-C'ing gclient has to manually terminate hung SVN processes, so this
introduces no extra data loss or hazard. stracing a hung SVN process shows that
it is indeed hanging on TCP reads after receiving a SIGINT, implying there is an
underlying but in the SVN binary.

Review URL: https://chromiumcodereview.appspot.com/14759006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@198205 0039d316-1c4b-4281-b951-d872f2087c98