Add glnx_open_anonymous_tmpfile_full() allowing you to specify the directory

This is useful if you need the file to be on a particular filesystem. In particular, flatpak wants this to make tempfiles on /tmp for things we need to write during flatpak run, such as the libseccomp output fd.

We've had "flatpak run" stop working in low disk situations without this, so its nice to be able to fix it.