This allows `termio.Exec` to track processes spawned via
`FlatpakHostCommand`, finally allowing Ghostty to function as a
Flatpak.
Alongside this is a few bug fixes:
* Don't add ghostty to PATH when running in flatpak mode since it's
unreachable.
* Correctly handle exit status returned by Flatpak. Previously this was
not processed and contains extra status bits.
* Use correct type for PID returned by Flatpak.
The variant format string `^aay` is said to be equivalent to
g_variant_new_bytestring_array. Given that no length parameter is
provided, g_variant_new assumed a null-terminated array, but the array
constructed by the code was not, causing a crash as glib exceed the read
boundaries to copy arbitrary memory.
This commit replaces the array construction code to use its arena
equivalents instead of trying to build one using glib, and make sure
that the resulting array is null-terminated.
