libc: Fix some -Wunused-parameter.
libc: Fix -Wmissing-field-initializer for FAKE_FILE usages.

While here, use PTHREAD_MUTEX_INITIALIZER for _fl_mutex initialization.
libc: __atfork() is only used by the WR() in this file, so make it static.

Also mark it __used.

This silences a -Wmissing-prototypes warning.