Make readlink(1) compliant with the POSIX-2004 (Issue 8).

POSIX 2004 requires that if a given file name to readlink(1)
doesn't refer to a symlink, an error be printed.

That conflicts with historic usage on NetBSD where readlink is
silent in that case (except for the exit status) -- rather than
altering our default behaviour, look for POSIXLY_CORRECT in the
environment, and issue an error only in that case (just means
switching between -q and -v as the default noise option).

While here, generate usage if no file args are given - POSIX
requires at least 1 (it actually specifies exactly 1) and while
we can extend that without issues to allow more than one, allowing
0 args (just because we do that for stat(1) and readlink(1) is
the same code) makes no sense at all for readlink - stdin cannot
possibly be a symbolic link.

Also, in the case of stat(1) if we do have no args, and the
fstat(STDIN) fails for some reason, don't claim that it is
lstat (or stat, depending upon -L) which failed:
stat: (stdin): lstat: Bad file descriptor
makes no sense at all. This can occur with something like
stat <&-
which is kind of obscure, but it is easy to DTRT.

No pullups planned.

branches: 1.6.4;
readlink: sort SEE ALSO

Add an Xr to realpath(1) (which does much the same as readlink -f).
Try not to forget the damn comma!

Remove dot at end of SEE ALSO.

readlink(1): Add italic correction.

readlink(1): Add missing .El, use non-random .Bl -width

Divide stat.1 into stat.1 (now only includes stat(1)) and readlink.1

Apologies to cvs commit purists, but making this division required
line by line reading of the man pages, and I simply could not resist
also correcting some errors, addressing some omissions, improving some
wording ... all at the same time.