all: use lowercase log messages

Go style recommends that log messages and error strings should not be
capitalized (unless beginning with proper nouns or acronyms) and should
not end with punctuation.

This change normalizes all zerolog .Msg() and .Msgf() calls to start
with lowercase letters, following Go conventions and making logs more
consistent across the codebase.

hscontrol/poll.go

@@ -308,7 +308,7 @@ func (m *mapSession) writeMap(msg *tailcfg.MapResponse) error {
 		if f, ok := m.w.(http.Flusher); ok {
 			f.Flush()
 		} else {
-			m.log.Error().Caller().Msg("ResponseWriter does not implement http.Flusher, cannot flush")
+			m.log.Error().Caller().Msg("responseWriter does not implement http.Flusher, cannot flush")
 		}
 	}