- Add missing `sys/ttydefaults.h` include to `config.c`
 - Restrict usage of `error.h` to glibc and uclibc only
 - Avoid including `linux/if_tun.h` on non-glibc/uclibc to prevent
   musl kernel/libc header conflicts

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