ctdbd: Complain loudly if CTDB_DEBUG_HUNG_SCRIPT script isn't executable

ctdbd: Complain loudly if CTDB_DEBUG_HUNG_SCRIPT script isn't executable

This is quite easy to misconfigure by failing to set the execute bit
on the script.  Better to complain loudly.

This is a debugging facilty rather than core CTDB functionality, so it
doesn't need a subtle mechanism to disable it at run-time.  To disable
the designated script at run-time either edit it to put an "exit 0" at
the top or move it aside and symlink to /bin/true.

This is implemented by actually removing the code that checks that the
file exists and is executable.  The output from the shell when the
system() function fails is just as useful.

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