Adding mulitple-valued ENV variables option to mod_auth_mellon

In our  SSO (MS ADFS) environment we perform authorization most of the time passing IDP attributes to applications as apache ENV variables. Current mod_auth_mellon version sets these variables as single-value suffixing variable name with digits (VAR_0 = val1 , VAR_1 = val2 .. etc) which is not very practical for the above mentioned purpose .... 
Proposed patch allows - optionally - to set  multi-valued variables (VAR = val1;val2; .. etc) making mod_auth_mellon act same way the shibboleth apache module does:
 https://wiki.shibboleth.net/confluence/display/SHIB2/NativeSPAttributeAccess

Please consider including proposed patch in future mod_auth_mellon releases.

Jarek
since `apr_pstrcat(r->pool, varname_prefix, varname, NULL)` is used several times in this function, I think it makes sense to just calculate it once.