chgserver: make it a core module and drop extension flags It was an extension just because there were several dependency cycles I needed to address. I don't add 'chgserver' to extensions._builtin since chgserver is considered an internal extension so nobody should enable it by their config.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL