Disable connectat/bindat with AT_FDCWD in capmode

Previously it was possible to connect a socket (which had the
CAP_CONNECT right) by calling "connectat(AT_FDCWD, ...)" even in
capabilties mode. This combination should be treated the same as a call
to connect (i.e. forbidden in capabilities mode). Similarly for bindat.

Disable connectat/bindat with AT_FDCWD in capabilities mode, fix up the
documentation and add tests.

PR: 222632
Submitted by: Jan Kokemüller <jan.kokemueller@gmail.com>
Reviewed by: Domagoj Stolfa
MFC after: 1 week
Relnotes: Yes
Differential Revision: https://reviews.freebsd.org/D15221