When the back-end refuses initialisation, return diagnostics

When the back-end refuses initialisation, return diagnostics
in a UserInfo-1 external in the Init response's User-information
field, in accordance with Z35.90 Implementor Agreement 5
(Returning diagnostics in an InitResponse) at
	http://lcweb.loc.gov/z3950/agency/agree/initdiag.html

(It's bothered me for literally about six and a half years that
YAZ didn't do this -- right back to when I made the Index+/Z39.50
gateway for SSL in 1997.)