glib: add APIs to load/save to/from file descriptors
This is useful when all you have is a file descriptor, e.g. when going through a portal to open or save; and also when using O_TMPFILE tempfiles to pass between processes.
This is useful when all you have is a file descriptor, e.g. when going through a portal to open or save; and also when using O_TMPFILE tempfiles to pass between processes.