Added a user defined title to spice netlists

[G]

The attached patch adds:
1. A new symbol called spice-title which has a single value attribute
2. support in the spice-sdb netlist backend to use the value of the spice-title device as the first line of the spice file.

This is the first of several changes I am going to be offering which will hopefully culminate in a working netlist backend to generate netlists to work with ngspice w/ xspice. If I can do it this should result in full support for digital and hybrid simulation in geda/gaf via ngspice w/ xspice.

I didn't like haveing ngspice display the full xorn command line as the netlist title in interactive mode. This seemed like a good place to start as while I have a long history working with C/C++ this is my first time using scheme.

To apply the patch go to the root of the geda/gaf tree (i.e. the directory containing the HACKING file) and use patch -p 1 < <path-to-the-patch>

This will add one symbol to the symbols directory and patch the gnetlist-legacy/scheme/gnet-spice-sdb.scm file

It patched cleanly for me against the current master branch from the git repository.

To use the functionality add a spice-title block device to your schematic. Set the value attribute to the tile you want spice to use. Then generate the netlist like normal.