Previously, the code in `convert_path()` could not handle keys longer
than `G_MAXINT`, and would overflow if that was exceeded.

Convert the code to use `gsize` and `g_memdup2()` throughout, and
change from identifying the position of the final slash in the string
using a signed offset `i`, to using a pointer to the character (and
`strrchr()`). This allows the slash to be at any position in a
`G_MAXSIZE`-long string, without sacrificing a bit of the offset for
indicating whether a slash was found.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Helps: #2319