background start-up: use PID file to signal success

This change introduces the use of elasticsearch's PID file to signal a
successful start-up. The start script bin/elasticsearch will wait a
configurable amount of time (default: 120s) for the PID file to appear. After
this timed out the script will print an error and return an error code.

Note that if no PID file was specified on the command line the script will now
use 'mktemp' to create a "private" PID file (only if ES is to be started in the
background, though). The command line help was updated correspondingly.

Also, the PID file generation code has been moved from the beginning of the
bootstrap main() method to the end.

Fixes #8652

Signed-off-by: Thilo Fromm <github@thilo-fromm.de>