libbuxton: use wrapper functions for get_name() and get_group()

Some API functions are useful for both public and private code;
buxton_get_group() and buxton_get_name() are two examples.

Since private code should not be using the public API, make these two
functions wrappers for internal equivalents.

Also, use get_group() and get_name() where appropriate.
build: cleanups for libbuxton linkage

Both bt-daemon and libbuxton-security no longer use the public
libbuxton API, so remove the linkage.