[1/3] Add mostly Linux-compatible file sealing support

File sealing applies protections against certain actions
(currently: write, growth, shrink) at the inode level. New fileops are added
to accommodate seals - EINVAL is returned by fcntl(2) if they are not
implemented.

Reviewed by: markj, kib
Differential Revision: https://reviews.freebsd.org/D21391