cred: Hide internal flag CRED_FLAG_CAPMODE

This flag is used in field 'cr_flags', which is never directly visible
outside the kernel. That field is however exported through 'struct
kinfo_proc' objects (field 'ki_cr_flags'), either from the kernel via
sysctls or from libkvm, and is supposed to contain exported flags
prefixed with KI_CRF_ (currently, KI_CRF_CAPABILITY_MODE and
KI_CRF_GRP_OVERFLOW, this second one being a purely userland one
signaling overflow of 'ki_groups').

Make sure that KI_CRF_CAPABILITY_MODE is the flag actually exported and
tested by userland programs, and hide the internal CRED_FLAG_CAPMODE.
As both flags are currently defined to the same value, this doesn't
change the KBI, but of course does change the KPI. A code search via
GitHub and Google fortunately doesn't reveal any outside uses for
CRED_FLAG_CAPMODE.

While here, move assignment of 'ki_uid' to a more logical place in
kvm_proclist(), and definition of XU_NGROUPS as well in 'sys/ucred.h'
(no functional/interface changes intended).

Reviewed by: mhorne
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46909