No need to set if_mtu since it automatically updated by ether_ifattach().
Use  if_initbaudrate() to set baudrate.
Add IFCAP_LINKSTATE to if_capabilities.

Submitted by:	David C Somayajulu <davidcs@freebsd.org>
Approved by:	George Neville-Neil <gnn@freebsd.org>
Allow the use of lowercase 'yes'

PR:		bin/178422
Submitted by:	Garrett Cooper <yaneurabeya@gmail.com>
Make errbuf optional, so if a caller is not interested in an error
message she can pass NULL (procstat(1) already does this).

MFC after:	2 weeks