wifi-menu: Use proper quoting syntax for hexadecimal keys

I noticed that profiles generated by `wifi-menu -o` don't use the "Special quoting rules" for hexadecimal keys given at the end of [netctl.profile(5)](https://github.com/joukewitteveen/netctl/blob/master/docs/netctl.profile.5.txt#L446).  This prefixes both auto-generated and manually-entered hexadecimal keys with `\"`.

This looks like the same wifi-menu issue mentioned in #67.