Use pid to shutdown the p4d process.
1. Make shutdown more standard using kill/SIGTERM.
2. Make shutdown more reliable; no need to be logged in, insluated from P4AUTH, etc.
3. Use the now-built-in pid mechanism.
4. On shutdown, make it so the script doesn't return until the p4d process is well
and truly down.
This was implemented with some backward-compatibilty features to simplfy SDP
1. Though the p4_N.vars file should add the required '--pid-flag' to P4D_FLAGS,
this new p4d_base checks and adds it if it is missing.
2. The old 'p4 admin stop' logic is retained as a new 'admin_stop' function, and
is used if the server.pid file does not exist when stop is called.