vfs: fix SIGHUP killing serve instead of flushing directory caches

Before, rclone serve would crash when sent a SIGHUP which contradicts
the documentation - saying it should flush the directory caches.

Moved signal handling from the mount into the vfs layer, which now
handles SIGHUP on all uses of the VFS including mount and serve.

Fixes #8607

vfs/sighup_unsupported.go

@@ -0,0 +1,10 @@
+//go:build plan9 || js
+
+package vfs
+
+import (
+	"os"
+)
+
+// NotifyOnSigHup makes SIGHUP notify given channel on supported systems
+func NotifyOnSigHup(sighupChan chan os.Signal) {}