Revert "remove syscalls.c and define the test calls as macros"

This reverts commit e79a9c196d57393599847d40b15efc2464d5d57c.

Conflicts:
	radvd.h

The way automake builds device-linux.c means that RADVD_TEST will
not be defined they way we'd expect, so add back the syscalls.c
layer and don't try to define radvd_* system/glib calls with
macros.