cfg: Fix behavior of cfg_get(0, "a") when a key "a.b" exists.

cfg: Fix behavior of cfg_get(0, "a") when a key "a.b" exists.

The intent of cfg_get_*(0, "a") is to get the first value of a key with
the exact name "a", but in the presence of a key with a longer name, e.g.
"a.b", it would return the value of that key instead.

This file really needs a unit test!  I'm really not certain that
the fix didn't break other things (e.g. cfg_has_section()).