Merge from vendor branch TNF:

Merge from vendor branch TNF:
Import ruby19-posix-spawn-0.3.6 as devel/ruby-posix-spawn.

fork(2) calls slow down as the parent process uses more memory due to
the need to copy page tables.  In many common uses of fork(), where it
is followed by one of the exec family of functions to spawn child
processes (Kernel#system, IO::popen, Process::spawn, etc.), it's
possible to remove this overhead by using the use of special process
spawning interfaces (posix_spawn(), vfork(), etc.)

The posix-spawn library aims to implement a subset of the Ruby 1.9
Process::spawn interface in a way that takes advantage of fast process
spawning interfaces when available and provides sane fallbacks on
systems that do not.