MFC r306657, r306673, r306726, r307737, r309366, r310135, r323990, r324414

r306657:
libcapsicum: introduce Capsicum helpers

Capsicum helpers are a set of inline functions which goal is to reduce
duplicated patterns used to Capsicumize applications.

Reviewed by: cem, AllanJude, bapt, ed, emaste
Differential Revision: https://reviews.freebsd.org/D8013

r306673:
libcapsicum: limit stderr

Don't limit stdout twice, instead limit stderr.

Pointed out by: rpokala@

r306726:
Add man pages for Capsicum helpers.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D8154

r307737:
Fix few sentence in the man page.

Pointed out by: wblock

r309366:
capsicum_helpers: Squash errors from closed fds

Squash EBADF from closed stdin, stdout, or stderr in caph_limit_stdio().
Any program used during special shell scripts may commonly be forked
from a parent process with closed standard stream. Do the common sense
thing for this common use.

Reported by: Iblis Lin <iblis AT hs.ntnu.edu.tw>
Reviewed by: oshogbo@ (earlier version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8657

r310135:
capsicum_helpers: Add LOOKUP flag

Add a helper routine for opening a directory that is restricted to being
used for opening relative files as stdio streams.

I think this will really help basic adaptation of multi-file programs to
Capsicum. Rather than having each program initialize a rights object and
ioctl/fcntl arrays for their root fd for relative opens, consolidate in the
logical place.

Reviewed by: oshogbo@
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8743

r323990:
capsicum_helpers: Add SEEK to default stdio rights set

PR: 219173
Sponsored by: Dell EMC Isilon

r324414:
capsicum_helpers: Add EVENT to default stdio rights set

Without it, calling caph_limit_stdio(3) breaks Irssi.

Reviewed by: oshogbo
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12622