git_config_parse_key() implements the validation and downcasing of
<section> and <variable> in "<section>[.<subsection>].<variable>"
configuration variable name. Move it (and helpers it uses) a bit up
so that it can be used by git_config_parse_parameter(), which is
used to check configuration settings that are given on the command
line (i.e. "git -c VAR=VAL cmd"), in a later patch.