Re-enable incremental mode in child process on Darwin

[ivmai]

This issue (feature request) is to resolve TODO (Re-enable incremental mode in child) mentioned in:

bdwgc/os_dep.c

Line 4580 in ba2861e

/* TODO: Re-enable incremental mode in child. */

Related issue #103

I think we should also add an API to disable automatic of re-enabling of incremental mode in child (because some clients call exec right after fork and re-enabling of the incremental mode requires resources usage on some platforms - reopen files, recreate a dedicated thread, etc.).