debug: add support for per debug-class logfiles

debug: add support for per debug-class logfiles

This adds support for per debug-class logfiles to the function parsing
the "log level" option.

The enhanced syntax is:

  log level = CLASS:LEVEL[@PATH] [CLASS:LEVEL[@PATH] ... ]

Eg

  log level = full_audit:1@/var/log/audit.logfile

While the option is already parsed and stored in in the dbgc_config[]
array, the feature is still effectively disabled, as
reopen_logs_internal() still doesn't open the per debug-class logfiles.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>