keyfile: avoid needless allocations on file load

keyfile: avoid needless allocations on file load

When loading a key file, the keys and values of individual lines
are allocated once when copied and trimmed from the parse buffer
and allocated/copied again when added to the lookup map.

This commit avoids the second pair of allocations by introducing
a new function g_key_file_add_key_value_pair that gives the
lookup map direct ownership of the key and value copied from the
parse buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=650211