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.
You're still allowing identifiers that start with digits.  This is, at least, inconsistent with bundle.
Why not use the same regex as bundle identifiers, that you quoted in the bug ?