gram.y: This looks like a bug, but not sure...

I ran into some crashes while writing unit tests and found these
lines here.  I *think* the two lines under the else need to be
wrapped in curly braces.
gram.y: fix memory problems exposed by unit tests

Interfaces were not properly freed when the config file fails to parse.
add print_safe_buffer
configure.ac: add PKG_CHECK_MODULES([CHECK])
add unit tests
add dnssl and rdnss checks
add test_add_ra_header
add test_prefix