rust-chg: extract signal handlers from chg/procutil.c
abortmsgerrno() and debugmsg() are removed, and the public interface instead
returns success/error status. Since signal handlers can't propagate errors,
the result of kill() is just ignored.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL