dbg: Better logging, -d runcmd_trace_file=FILEPATH support

dbg.v

@@ -1,6 +1,15 @@
 module runcmd
 
+import os
+
 @[if runcmd_trace ?]
 fn printdbg(s string) {
-	eprintln('runcmd[pid=${v_getpid()}]: ${s}')
+	text := 'runcmd[pid=${v_getpid()}]: ${s}'
+	eprintln(text)
+	trace_file := $d('runcmd_trace_file', '')
+	if trace_file != '' {
+		file := os.open_append(trace_file) or { return }
+		file.write_string(text + '\n') or {}
+		file.flush()
+	}
 }