MFC r331274

Extend the description of ALTQ to call it a system which is a framework in
altq(4) to match altq(9). This makes preserving the history section as the
author of ALTQ easier in the history section, rather than calling it a framework
in the description & a system in the history.
Add a history section to altq(4) and extend the history section in altq(9)

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix mandoc warnings/errors.

MFC after: 1 week

Remove ifq_drops from struct ifqueue. Now queue drops are accounted in
struct ifnet if_oqdrops.

Some netgraph modules used ifqueue w/o ifnet. Accounting of queue drops
is simply removed from them. There were no API to read this statistic.

Sponsored by: Netflix
Sponsored by: Nginx, Inc.

Garbage collect man page reference to IFF_NEEDSGIANT.

Add a minimal altq.4 manpage to tell about the kernel options and where to
find more information. Also move the "SUPPORTED DEVICES" section from altq.9
to altq.4, where is belongs.

Requested by: green
MFC after: 2 days

dc(4) is now supported as well. While here sort the list and remove awi,
which isn't yet supported.

Document supported devices here (for lack of a better place). Users
interested in ALTQ are likely to type in "man altq" at some point and maybe
they will scroll down to the bottom as well.

MFC after: 3 days

mdoc(7) and english grammar makeover.

Submitted by: ru

Add altq(9) explaining about the IFQ_* and IFQ_DRV_* macros in if_var.h and
how to convert drivers.

Obtained from: NetBSD (with changes)
Reviewed by: josef