Add dup3(), based on F_DUP2FD and F_DUP2FD_CLOEXEC fcntls.

I removed functionality not proposed for POSIX in Austin group issue #411.
A man page (my own) and test cases will follow in later commits.

PR:		176233
Submitted by:	Jukka Ukkonen
Add man page dup3(3).
Add tests for dup3().