Redmine#6063: Context name validation is too strict

see https://dev.cfengine.com/issues/6063

brings back ec62d6798ede560f496f9073c6d82ff72b1ec3cb and adds logging and better validation with a regex, based on what's in `cf3lex.l`.  `[\200-\377]` are excluded from the regex right now.
This shouldn't work.