dirfd: Use better and faster random algorithm for gen_temp_name()

I was looking at ostree performance, and a surprising amount of
time was spent in `glnx_gen_temp_name()`.  We end up calling it
from the main loop, and the iteration here shows up in my perf
profiles.

The glibc algorithm here that we adopted is *very* dated; let's
switch to use `GRand`, which gives us a better algorithm.

It'd be even better of course to use `getrandom()`, but we should do that in
glib at some point.

While I had the patient open, I extended the charset with lowercase, to better
avoid collisions.

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