Wrong net numbering in spice, spice-sdb and switcap netlisters

[Vladimir Zhbanov]

An archive with an example schematic and Makefile containing
commands to demonstrate this bug is attached.

You can see that after netlisting, two nets having no netname=
attribute will have the same net numbers as two others which have
those attributes, while the latter nets have no connection to the
former ones.

I've also checked all the other backends (excluding cascade
and makedepend, which fail on this test schematic) and
found them all working well with net renaming/renumbering.

Unlike the two spice backends, switcap fails with the following
output:
  ***** ERROR *****
  There is a net name collision!
  ...
  and so on

The bug is present in the current git master HEAD, commit b63aa12.