When running --fork'ed, selectively mute some messages in child processes
(those messages that are expected to be printed by the main process as well,
such as on warning/error conditions that are almost certainly common to all
processes).
John's global variables exported from john.c to other source files (currently
this is just john_main_process).
Moved and renamed sig_pids from signals.[ch] to john_child_pids in john.[ch].
Introduced john_child_count.
Added john_main_process, john_child_count, and john_child_pids to best.c to
repair "make generic" builds, which were broken with recent commits