Add uci option mppe which makes the mppe parameter configurable;
the default value is required,no40,no56,stateless as before.

Add uci option logwtmp; when enabled updates wtmp when users
connect and disconnect

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

- Ship a net/ppp_defs.h replacement header since musl does not provide one
   but pppd/pppd.h provided by pppd is needing it.
 - Replace deprecated MSG_TRYHARD flag with the proper MSG_DONTROUTE one
 - Unconditionally include string.h in bcrelay to avoid undeclared memset()
   and strcpy() warnings.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

Signed-off-by: Luka Perkov <luka@openwrt.org>