meson: rework generation of file system lookup helpers

meson: rework generation of file system lookup helpers

Previously, the gperf table was the main "source of truth", and additional
information (e.g. which file system names are obsolete) was scattered in
various files. We would then parse the gperf file, using python, awk, grep, and
bash, and use the results in various ways. This is hard to understand and
maintain. Let's replace all of this with a single python script that generates
the requested outputs as appropriate.

$ diff -u <(git show @{u}:src/basic/filesystems-gperf.gperf) \
          <(src/basic/filesystem-sets.py gperf)
shows that the outputs are the same except for comments.

Similarly, 'src/basic/filesystem-sets.py fs-type-to-string' and
'src/basic/filesystem-sets.py filesystem-sets' can be used to view
the generated code.

The check that the kernel doesn't define any new file system magics is
converted into a normal test. It doesn't seem necessary to fail the build
when that happens.