util: remove use hardcoded TC path

util: remove use hardcoded TC path

Allow virCommand to find 'tc' in $PATH. This command is only used
when running privileged in which case both 'bin' and 'sbin' dirs will
be in $PATH, so virFindFileInPath will do the right thing to find it.

Since there are no longer any optional programs, only optional test
programs, the meson variables can be renamed and simplified at this
point.

The "TC" constant is defined in the header to match the pattern used
for the other firewall tool names.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>