1. Add $main::bumpexisting variable which allows a new connection for a user

1. Add $main::bumpexisting variable which allows a new connection for a user
call to disconnect an existing connection with the same call. The default for
this variable is 'true' (1). This means that existing behaviour has changed.
set/var $main::bumpexisting = 0 to return to original behaviour.
2. Remind a user every hour (as default) that they have new messages.
set/var $DXCommandmode::msgpolltime = 3*3600 to set it to 3 hours.
3. Allow some substitutions in set/prompt (call, date, time and
cluster call) and add dxspider just before the > as standard.
4. Add $main::allowdxby variable so that sysops can switch off the
'DX BY <othercall' command. It is set to 0 (ie disabled) by default. This
is for sysops who don't want to bump privilege to allow it. That still works
as before.
5. Fix the bug in 'dx by <call>' that causes only the first word of the
comment to be recorded.