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
I don't think this will have the proper effect. The goal of the merge routine is to take whatever value is set in `add_cfg` if any value is set in the configuration parsed in `add_cfg`. Otherwise the value in `base_cfg` will be used.

The code as it is written allows a new configuration to set the flag to enabled, but will not allow it to be disabled. I think you probably want to use `-1` as the default value instead, and make sure to interpret that value as off.