MFC: file 5.21
MFH r230580:
Don't mention no longer supported ioctl commands.

MFH r274083:
convert .Nm to proper .Xr's...

MFH r275298:
Cross reference tap(4) and tun(4) and include a short explanation as
to how they differ.  This will assist users in selecting which interface
is more appropriate for their purposes.

No objection:	glebius (r230580), jmg (r274083)
Approved by:	grog (co-mentor)
MFC r267897: Update Makefile to reflect file update.