Update mbuffer to 20150412

20150412:
- fix: watchdog cannot do its job if it is cleaned up to early
- fix: clear sa_flags and sa_mask before use, although ignored

20141227:
- added option --pid to print PID of current process
- updated usage to include option -W
- man page formatting fixes
- configure updates
- support compiling on systems that do not define PATH_MAX

(wiedi)