ctdbd: Backport use of external script to debug hung eventscript

ctdbd: Backport use of external script to debug hung eventscript

This is a cherry-pick from 6e68797af67bee36f2bad045f94806e7e98f27e9,
combined with several recent fixes:

  8507303b525d20c74e8ec4e7c4f5f275945cd3b6
    scripts: debug-hung-script.sh doesn't need functions/loadconfig
  501461cc3e132d4adee9e91b5d4513a26bae2846
    ctdbd: Remove debug_hung_script_ctx
  0581f9a84e58764d194f4e04064c2c5b393c348b
    ctdbd: Remove command-line option --debug-hung-script
  3400b2ed34b6eb9496eb55f1aab6f89d2952060d
    ctdbd: Complain loudly if CTDB_DEBUG_HUNG_SCRIPT script isn't executable
  9b0d56b16775aa16f33bdfdf831256e085fa3339
    ctdbd: Don't use a fixed length buffer for the hung script command

Signed-off-by: Martin Schwenke <martin@meltin.net>
Cherry-pick-from: b86270fae7fd9f8a7a718e15d8c7436a918f28c4