wpa_supplicant: Move wpa_config_get_line() into utils

wpa_supplicant: Move wpa_config_get_line() into utils

The function wpa_config_get_line() is used by the wpa_supplicant config
file parser to retrieve the next non-comment non-blank line. We'll need
the same kind of functionality to implement the file-based external
password backend, so as a preparatory step this commit extracts the
function into its own standalone file in the utils package.

No functional changes are expected from this commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>