Detect running under Valgrind, and turn off Landlock/Seccomp in
order to make the tracker-extract-3 process traceable.

This obviously errs on the side of user friendliness, making it
possible to just run:

$ valgrind --trace-children=yes /usr/libexec/tracker-miner-fs-3

To get reports for both tracker-miner-fs-3 and tracker-extract-3
in a single run, or:

$ valgrind /usr/libexec/tracker-extract-3 -f /path/to/file

To get Valgrind reports of tracker-extract-3 extracting metadata
from an individual file. The current alternative to trace memory usage
in the tracker-extract-3 process is to disable Landlock/Seccomp
functionalities at build time (i.e. requiring a development environment),
this is not great if we have to ask users to run Valgrind for us.

These checks are done prior to dealing with any untrusted data, so
it is implausible that exploits in the filesystem could pre-emptively
alter the environment to turn off these features. And optimistically
the virtualized environment while running under Valgrind should throw
off attempts from exploits to take advantage of memory handling
mishaps. These caveats should not be completely ignored though, if
you have untrusted files and want to profile memory usage of
tracker-miners processes.

An additional measure adopted by tracker-extract-3 when running
under Valgrind is disabling the per-file deadline, since Valgrind
may make things slow enough to challenge it.