Add dhcpcd_encode which encodes raw data back into a safe string.

Add dhcpcd_encode which encodes raw data back into a safe string.
This is used to convert wpa_supplicant safe strings into dhcpcd ones so the
display is consistent.
If a SSID has unsafe characters in and saving to wpa_supplicant.conf then
convert the SSID to a hex string.
A vis(3) shim has been added if libc does not provide one.