Rename "log" config directive to "message-store"

This is more explicit and removes the ambiguity with error logging.

config.in

@@ -1,2 +1,2 @@
 db sqlite3 /var/lib/soju/main.db
-log fs /var/lib/soju/logs/
+message-store fs /var/lib/soju/logs/

config/config.go

@@ -109,7 +109,7 @@ func parse(cfg scfg.Block) (*Server, error) {
 			if err := d.ParseParams(&srv.SQLDriver, &srv.SQLSource); err != nil {
 				return nil, err
 			}
-		case "log":
+		case "message-store", "log":
 			var driver string
 			if err := d.ParseParams(&driver, &srv.LogPath); err != nil {
 				return nil, err

doc/soju.1.scd

@@ -133,10 +133,12 @@ The following directives are supported:
 	  strings, see:
 	  <https://pkg.go.dev/github.com/lib/pq#hdr-Connection_String_Parameters>.
 
-*log* fs <path>
+*message-store* fs <path>
 	Path to the bouncer logs root directory, or empty to disable logging. By
 	default, logging is disabled.
 
+	(_log_ is a deprecated alias for this directive.)
+
 *http-origin* <patterns...>
 	List of allowed HTTP origins for WebSocket listeners. The parameters are
 	interpreted as shell patterns, see *glob*(7).