m_spanningtree UID handler: Fix parsing user modes

Spotted by @ShutterQuick
Issue #532
Merge pull request #523 from SaberUK/master+server-notice

Add method for writing server notices.
Replace some C-isms with C++-isms.

* 'const char*' to 'const std::string&'.
* snprintf to std::string concatenation.
* Replace duplicated OneOfMatches with InspIRCd::MatchMask.