libctdb: add logging infrastructure

libctdb: add logging infrastructure

This is based on Ronnie's work, merged with mine.  That means
errors are all my fault.

Differences from Ronnie's:
1) use syslog's LOG_ levels directly.
2) typesafe arg to log function, and use it (eg stderr) in helper function.
3) store fn in ctdb context, and expose ctdb_log_level directly thru API.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>