Use a GQueue for zephyr plus related cleanup

* Make `_Z_InputQ->complete` a boolean, as that's all it's used as.
* Use GLib allocation functions for `Z_InputQ` members.
* Use a `GQueue` for zephyr input.
* Use `*_find_custom` functions for input queue search.

Testing Done:
Compile only.

Reviewed at https://reviews.imfreedom.org/r/253/