Eventscripts - generalise TCP port checking plus new nmap-based checker

Eventscripts - generalise TCP port checking plus new nmap-based checker

Split the netstat-specific parts of ctdb_check_tcp_ports() into new
function ctdb_check_tcp_ports_netstat().

Implement new ctdb_check_tcp_ports_nmap() function that uses
"nmap -PS" to check if the desired ports are listening.

ctdb_check_ctdb_ports() now uses new configuration variable
CTDB_TCP_PORT_CHECKERS to decide which port checkers to try.  Default
value is currently "nmap netstat".  If nmap is not found then this
will fall back to netstat.  This indicates that either nmap should be
installed or the default value of CTDB_TCP_PORT_CHECKERS should be
changed (in a configuration file) to avoid trying to use nmap.

Signed-off-by: Martin Schwenke <martin@meltin.net>