Allow memory unmapping in case of MPROTECT_VDB

Allow memory unmapping in case of MPROTECT_VDB

Issue #271 (bdwgc).

* allchblk.c [USE_MUNMAP && MPROTECT_VDB] (GC_has_unmapped_memory):
Remove function definition.
* os_dep.c [MPROTECT_VDB && USE_MUNMAP] (GC_dirty_init): Likewise.
* doc/README.macros (USE_MUNMAP): Update the description.
* include/private/gc_priv.h [MPROTECT_VDB && USE_MUNMAP]
(GC_mprotect_dirty_init, GC_has_unmapped_memory): Remove prototype.
* include/private/gcconfig.h [USE_MUNMAP && GWW_VDB] (MPROTECT_VDB):
Do not undefine; remove TODO; remove comment.
* include/private/gcconfig.h: Remove comment that PARALLEL_MARK does
not cause undefine MPROTECT_VDB.
* os_dep.c [MPROTECT_VDB && !USE_MUNMAP] (GC_mprotect_dirty_init):
Remove macro.
* os_dep.c [MPROTECT_VDB] (GC_mprotect_dirty_init): Rename to
GC_dirty_init.
* tests/test.c [!GC_DISABLE_INCREMENTAL && (TEST_DEFAULT_VDB
|| !DEFAULT_VDB)] (main): Call GC_enable_incremental() even in case of
both MPROTECT_VDB and USE_MUNMAP are defined.