Limit the number of door server threads in mountd(1m)
Currently, the mountd(1m) uses door server threads in the global pool with the default configuration. It means the number of the door server threads is unlimited and the stack size for each such thread is 1MB (mountd is 32-bit app).
Under some circumstances (especially, when mountd was not linked with libumem, see #4990 for details) we saw thousands door threads consuming significant part of the virtual memory and causing the memory exhaustion.
With the #4990 fixed the issue is not so hot, but it still would be great to have the number of door server threads limited, so they do not consume all of the memory. Also, the stack size for such threads should be limited too.