[PATCH/RFC] use-sasl=true in --without-sasl builds: make that a fatal error?

Currently, if use-sasl=true is set in svnserve.conf but svnserve was
compiled without SASL support, SASL is silently not used. That's
actually documented:

  svnserve.conf:
    [sasl]
    ### This option specifies whether you want to use the Cyrus SASL
    ### library for authentication. Default is false.
    ### This section will be ignored if svnserve is not built with Cyrus
    ### SASL support; to check, run 'svnserve --version' and look for a line
    ### reading 'Cyrus SASL authentication is available.'
    # use-sasl = true

But documentation notwithstanding, it seems like a misfeature. Should
we change this, so --without-sasl builds will error out if use-sasl=true
is set? The patch would be simple enough (attached).

Cheers,

Daniel

P.S. The 'password-db' setting is in the same boat: it's ignored
when SASL is enabled, and documented this way. However, I'm not
convinced a change to that setting's handling is needed.