MFC r332389, r3342331, r334343, and partially r339338

This commit backports some new features from iflib in 12 back to stable/11:

r332389 - Split out flag manipulation from general context manipulation in iflib
r334231 - Add IFLIB_ADMIN_ALWAYS_RUN flag
r334343 - hold context lock across detach
r339338 - (Other updates to iflib that were included in this revision)

This commit doesn't include mergeinfo for r339338 since this only includes part
of it. I'm reminded of advice given to keep MFC-ability in mind when committing
large changes in the future.

Reviewed by: marius@ (change content)
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D19949