Hard for users to get Undo backend working

[gareth-rees]

The Undo backend relies on the udbserver executable (for example, udbserver_x64) being on the path, but this does not happen if you download UDB and run make install, because the installer only puts symlinks to live-record and udb into the bin/ installation directory, leaving the other files in lib/udb-6.7.1/ or wherever.

Possible solutions are:

1. Document what users need to do to get it working.
2. Update UDB's make install so that it puts symlinks to the udbserver executables into the bin/ installation directory.
3. Deduce the lib/udb-6.7.1/ directory by using the Go equivalent of dirname $(readlink $(which live-record)).

[chimicus]

I believe that we should do both 1 and 3: I wouldn't touch make install for the main product as this specific issue affects only one integration, not the whole product.