Support clnt_raw's use of FD_SETSIZE as a fake file descriptor.

Accomplish this by allocating space for it in __svc_xports and allowing
it to be registered. The failure to allocate space was causing an
out-of-bounds read in svc_getreq_common(). The failure to register
caused PR 211804.

The bug was found with CHERI bounds checking.

PR: 211804
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Reviewed by: ngie
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D10528