Implement support for binary to requesting specific stack size for the
initial thread. It is read by the ELF image activator as the virtual
size of the PT_GNU_STACK program header entry, and can be specified by
the linker option -z stack-size in newer binutils.

The soft RLIMIT_STACK is auto-increased if possible, to satisfy the
binary' request.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week