Add infrastructure for building numeric namefield filters

Currently, all the CLI helpers in qlang.py and cli.py assume that all
namefields are string, which leads to various breakage in case the're
actually not.

To improve the flexibility of the helpers, we add a bit of
infrastructure for accepting so called "numeric" namefields; this is a
bit of a hack, as a proper fix would actually add QFT_* support to the
helpers, and case for example the regex/globbing on QFT_TEXT, etc. But
that's left for (eventual) later improvement.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>