kinfo_vmentry: report mappings of the SysV shm segments

Mark such mappings with the new flag KVME_FLAG_SYSVSHM.
Provide segment key in kve_vn_fileid, vnode never can back shm mapping.
Provide sequence number in kve_vn_fsid_freebsd11.

Reviewed by: markj (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D46959