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:             [[ https://bugs.freebsd.org/120233 | 120233 ]]
Submitted by:   Jukka Ukkonen
Approved by:    rwaston (mentor)
MFC after:      1 month