Make struct xinpcb and friends word-size independent.

Replace size_t members with ksize_t (uint64_t) and pointer members
(never used as pointers in userspace, but instead as unique
idenitifiers) with kvaddr_t (uint64_t). This makes the structs
identical between 32-bit and 64-bit ABIs.

On 64-bit bit systems, the ABI is maintained. On 32-bit systems,
this is an ABI breaking change. The ABI of most of these structs
was previously broken in r315662. This also imposes a small API
change on userspace consumers who must handle kernel pointers
becoming virtual addresses.

PR: 228301 (exp-run by antoine)
Reviewed by: jtl, kib, rwatson (various versions)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D15386