Problem restarting server

[BS-Labs]

Not really worth a pull request, I think: stop-docserver never un-sets global-acceptor, causing start-docserver to emit an error.

Steps to reproduce:

CL-USER> (docbrowser:start-docserver)
CL-USER> (docbrowser:stop-docserver)
CL-USER> (docbrowser:start-docserver)

Error: Server is already running