Add a new fo_fill_kinfo fileops method to add type-specific information to
struct kinfo_file.

• Move the various fill_*_info() methods out of kern_descrip.c and into the various file type implementations.
• Rework the support for kinfo_ofile to generate a suitable kinfo_file object for each file and then convert that to a kinfo_ofile structure rather than keeping a second, different set of code that directly manipulates type-specific file information.
• Remove the shm_path() and ksem_info() layering violations.

Differential Revision: https://reviews.freebsd.org/D775
Reviewed by: kib, glebius (earlier version)