Fix OPENTAR-53: "Tarantool ignores kill -15 on startup"
opentar53-sigterm: review fixes

Add constants for signal count. Rename. Coding style.
opentar53-sigterm: remove pid file on SIGTERM during the boot process
opentar53: review fixes

Set signal handler for all signals, not just SIGTERM.
Print signal name to the log on exit.
Merge branch 'opentar53-sigterm' into stable