base: differentiate KRB5 and other configurations on Windows
When the "KRB5_CONFIG" is unset on Windows, the registry values
HKLM\Software\Heimdal "config"
HKCU\Software\Heimdal "config"
are used. The migration of krb5_config to heimbase failed to
differentiate between KRB5_CONFIG, HX509_CONFIG, etc. The above
registry values are only for the KRB5_CONFIG.
This change permits the envvar name to be searched for in the
registry. For HX509_CONFIG the registry values
HKLM\Software\Heimdal "HX509_CONFIG"
HKCU\Software\Heimdal "HX509_CONFIG"
will be searched for configuration information.
Change-Id: I140945fa603d668d270eb5d740a11edc6fc121d7