Lib.pm: _fmsg(): directly jump to _msg() to avoid stack frame

_msg() needs to know how many stack frames to skip; if _fmsg() adds one
more, the debug messages cause the wrong function to be logged; e.g.

twitter-backup[400091]: log (Nullroute::Lib::_log): downloading entire timeline
twitter-backup[400091]: info (Nullroute::Lib::_info): Downloading 200 tweets

Use `goto &func` which does not have this effect.
Lib.pm, lib.bash: log full path of invoker