Starting with our conversions to mix backend-autodetection and selection
into a single variable (USE_GSSAPI, USE_HTTPS, USE_SHA1), we have
introduced a simple STREQUAL to check for "ON", which indicates that the
user wants us to auto-detect available backends and pick any one that's
available. This behaviour deviates from previous behaviour, as passing a
value like "yes", "on" or "true" will in fact be treated like a backend
name and result in autodetection failure.

Fix the issue by introducing a new function `SanitizeBool`. Given a
variable that may hold a boolean value, the function will sanitize that
variable to hold either "ON" or "OFF". In case it is not a recognized
boolean, we will just keep the value as-is. This fixes the above
described issue.