vfs: convert vfs options to new style

This also
- move in use options (Opt) from vfsflags to vfscommon
- change os.FileMode to vfscommon.FileMode in parameters
- rework vfscommon.FileMode and add tests

vfs/vfscommon/vfsflags_unix.go

@@ -0,0 +1,24 @@
+//go:build linux || darwin || freebsd
+
+package vfscommon
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// get the current umask
+func getUmask() int {
+	umask := unix.Umask(0) // read the umask
+	unix.Umask(umask)      // set it back to what it was
+	return umask
+}
+
+// get the current uid
+func getUID() uint32 {
+	return uint32(unix.Geteuid())
+}
+
+// get the current gid
+func getGID() uint32 {
+	return uint32(unix.Getegid())
+}