add subcommand "ximport", that is like "import" but directly access files in the datadir

so mox doesn't have to be running when you run it.
will be useful for testing in the near future.

this also moves cpuprof and memprof cli flags to top-level flag parsing, so all
commands can use them.

profile.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"log"
+	"os"
+	"runtime"
+	"runtime/pprof"
+)
+
+func memprofile(mempath string) {
+	if mempath == "" {
+		return
+	}
+
+	f, err := os.Create(mempath)
+	xcheckf(err, "creating memory profile")
+	defer func() {
+		if err := f.Close(); err != nil {
+			log.Printf("closing memory profile: %v", err)
+		}
+	}()
+	runtime.GC() // get up-to-date statistics
+	err = pprof.WriteHeapProfile(f)
+	xcheckf(err, "writing memory profile")
+}
+
+func profile(cpupath, mempath string) func() {
+	if cpupath == "" {
+		return func() {
+			memprofile(mempath)
+		}
+	}
+
+	f, err := os.Create(cpupath)
+	xcheckf(err, "creating CPU profile")
+	err = pprof.StartCPUProfile(f)
+	xcheckf(err, "start CPU profile")
+	return func() {
+		pprof.StopCPUProfile()
+		if err := f.Close(); err != nil {
+			log.Printf("closing cpu profile: %v", err)
+		}
+		memprofile(mempath)
+	}
+}