fix implicit definition of tolower()
fix implicit definition of read() and write()
eliminate unused variable warnings
fix implicit definition of pthread_create
fix type of heartbeat_monitor
fix accidental usage of 'error'.  use 'perror' instead
move definition of mime_types_add to prevent implicit declaration warning
move static definition of worker_thread into worker.c
fix implicit definition of clone(2)

It is necessary to #define _GNU_SOURCE to get a proper definition of
clone(2).  Also, deal with type warning fall out from this inclusion
(adjust type of worker_server)
fix implicit definition of pthread_create
move worker thread function declarations out of the public worker.h

Declare them as static and give them the correct type.
remove unused variables rc_cleanup and rc_heartbeat