merge fchmod() and fchmodat() guts, kill ancient broken kludge

The kludge in question is undocumented and doesn't work for 32bit
binaries on amd64, sparc64 and s390.  Passing (mode_t)-1 as
mode had (since 0.99.14v and contrary to behaviour of any
other Unix, prescriptions of POSIX, SuS and our own manpages)
was kinda-sorta no-op.  Note that any software relying on
that (and looking for examples shows none) would be visibly
broken on sparc64, where practically all userland is built
32bit.  No such complaints noticed...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>