chg: forward user-defined signals
SIGUSR1 and SIGUSR2 are reserved for user-defined behaviors. They may be
redefined by an hg extension [1], but cannot be easily redefined for chg.
Since the default behavior (kill) is not that useful for chg, let's forward
them to hg, hoping it got redefined there and could be more useful.
[1] https://bitbucket.org/facebook/hg-experimental/commits/
e7c883a465
chg: document why we send SIGHUP and SIGINT to process group
This makes the code more consistent - other signals are documented.