PPP, magic, enables building without PPP_MD5_RANDM support

PPP, magic, enables building without PPP_MD5_RANDM support

The only API difference with and without the PPP_MD5_RANDM support is the
availability of the random_bytes() function. Added a random_bytes()
function on top of magic() when PPP_MD5_RANDM support is not enabled,
thus allowing builds for both cases.

PPP_MD5_RANDM is still enabled by default (it was mandatory) if a protocol
using encryption is enabled, such as CHAP, EAP, or L2TP auth support.