MFC r267812 (hrs): Fix ifname normalization. ifconfig_IF_alias{es,N} did not
work if ifname has any of [.-/+].

While here, also perform the following to reduce diff size between major
branches for etc/network.subr:

MFC r266475 (rea): Fix warning messages after r252015

$alias used to hold alias number, but now it carries full variable name,
so messages were tuned to account for that.

Other fixes:
 - eliminate unneeded double spaces;
 - tell user where inet/inet6 keywords are expected to be.

Reviewed by:	hrs
Spotted by:	jhay
PR:		conf/191961