Version management updates for the new bob build script. There's now

Version management updates for the new bob build script. There's now
a fourth class of PuTTY version tags in addition to release,
snapshot and unidentified: we now have `Custom build r1234',
indicating a build made from that SVN revision in a context other
than that of a dated snapshot. The build script generates these when
it doesn't know what else to do; `unidentified builds' will now only
occur when you run nmake from the command line.

Also, the build script now generates sensible version data in the
installer to match this. So I _think_ we should now be set to use
bob to generate installer builds of the nightly snapshots, although
of course I'll have to wait until tomorrow to test one.

[originally from svn r7211]