The post_nm() validation function crashed when the first .Nm child node

The post_nm() validation function crashed when the first .Nm child node
was a non-text node.  Fix this by rewriting post_nm() to always set
the meta name to UNKNOWN when the name is missing or unusable.
While here, make MANDOCERR_NONAME an ERROR, as it usually renders
the page content unintelligible.

Bug reported by Maxim <Belooussov at gmail dot com>, thanks.
OpenBSD rev. 1.105