FIX: Disable incremental mode on Darwin if fork handling requested

FIX: Disable incremental mode on Darwin if fork handling requested

* doc/README.macros (NO_HANDLE_FORK): Replace back with HANDLE_FORK
and update.
* include/private/gcconfig.h (CAN_HANDLE_FORK): Define also for Darwin
in case of MPROTECT_VDB.
* misc.c (GC_set_handle_fork): Handle Darwin properly; add comment.
* os_dep.c (GC_dirty_init): Do not turn on GC_dirty_maintained for
Darwin if GC_handle_fork is on; add comment; remove FIXME.
* pthread_support.c (GC_fork_prepare_proc): Remove WARN for Darwin
(since GC_dirty_maintained is off in that case).
* pthread_support.c (GC_fork_child_proc): Remove comment (for Darwin).