Restrict valid keyword characters in metrics-db/-lib to [A-Za-z0-9-]+

A few invalid server descriptors containing a "?contact" line (with ? being non-ASCII bytes) confused stem's descriptor parser (see legacy/trac#8049 (closed)). These descriptors were not even valid according to dir-spec.txt:

KeywordChar ::= 'A' ... 'Z' | 'a' ... 'z' | '0' ... '9' | '-'

metrics-db/-lib currently don't consider these lines invalid, but only assume they contain an unknown keyword. They should instead reject these descriptors and prevent them from being included in tarballs.