Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Introduce a new F_DUP2FD command to fcntl(2), for compatibility with
Solaris and AIX.
fcntl(fd, F_DUP2FD, arg) and dup2(fd, arg) are functionnaly equivalent.
Document it.
Add some regression tests (identical to the dup2(2) regression tests).

PR: 120233
Submitted by: Jukka Ukkonen
Approved by: rwaston (mentor)
MFC after: 1 month

o Indent by tab.

o Add several regression tests for dup(2), dup2(2) and fcntl(F_DUPFD).

Obtained from: OpenBSD