- 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>