Fix gctest crash if configure --enable-handle-fork on Darwin

Fix gctest crash if configure --enable-handle-fork on Darwin
(Cherry-pick commit 457297d from 'release-7_6' branch.)

* include/private/gc_priv.h [!GC_DISABLE_INCREMENTAL] (GC_incremental):
Refine comment.
* include/private/gc_priv.h [!GC_DISABLE_INCREMENTAL]
(GC_dirty_maintained): Remove variable declaration.
* include/private/gc_priv.h [!GC_DISABLE_INCREMENTAL] (GC_dirty_init):
Change return type from void to GC_bool; update comment.
* mark.c [!GC_DISABLE_INCREMENTAL || CHECKSUMS] (GC_initiate_gc):
Replace GC_dirty_maintained with GC_incremental.
* mark.c [!GC_DISABLE_INCREMENTAL && PROC_VDB] (GC_push_conditional):
Likewise.
* mark.c [!GC_DISABLE_INCREMENTAL] (GC_push_next_marked_dirty): Likewise.
* misc.c [!GC_DISABLE_INCREMENTAL && !KEEP_BACK_PTRS]
(GC_enable_incremental): Likewise.
* os_dep.c [MPROTECT_VDB] (GC_remove_protection): Likewise.
* pthread_support.c [CAN_HANDLE_FORK && GC_DARWIN_THREADS
&& MPROTECT_VDB] (GC_atfork_prepare): Likewise.
* win32_threads.c [MPROTECT_VDB] (UNPROTECT_THREAD): Likewise.
* misc.c [!GC_DISABLE_INCREMENTAL] (GC_init): Set GC_incremental value
to the result of GC_dirty_init().
* misc.c [!GC_DISABLE_INCREMENTAL && !KEEP_BACK_PTRS]
(GC_enable_incremental): Likewise.
* misc.c [!GC_DISABLE_INCREMENTAL && !KEEP_BACK_PTRS]
(GC_enable_incremental): Set GC_incremental to true just before GC_init
call (to indicate the intention to turn on GC incremental mode).
* os_dep.c [!GC_DISABLE_INCREMENTAL] (GC_dirty_maintained): Remove
global variable.
* os_dep.c [GWW_VDB && !MPROTECT_VDB] (GC_gww_dirty_init): Define macro
to GC_dirty_init.
* os_dep.c [GWW_VDB && !MPROTECT_VDB] (GC_dirty_init): Remove function.
* os_dep.c [DEFAULT_VDB || MANUAL_VDB || MPROTECT_VDB || PCR_VDB]
(GC_dirty_init): Change return type to GC_bool; return true; remove
assignment of GC_dirty_maintained.
* os_dep.c [PROC_VDB] (GC_dirty_init): Replace ABORT with WARN and
return false if "pagedata" file open failed.
* os_dep.c [MPROTECT_VDB && DARWIN && CAN_HANDLE_FORK]
(GC_dirty_init): Replace GC_COND_LOG_PRINTF() with WARN()
which is called before return; refine WARN message.
* os_dep.c [MPROTECT_VDB && DARWIN] (GC_dirty_init): Return false if
GC_handle_fork; add TODO item to replace ABORT with WARN (and return
false).