The Agent ID is the string that we send to  web  servers  to
identify  ourself.  We default to no id at all, but some web
servers won't talk to a client unless it gives an acceptable
ID string.

The 1-char "flag" character should be '+' to enable  sending
the ID, and '-' or null to suppress it.

We don't yet know what are the best strings to send  to  the
various  web servers that require them.  This is an area for
further development ...