[mod_webdav] deprecated unsafe partial PUT compat

[mod_webdav] deprecated unsafe partial PUT compat

RFC 7231 forbids partial PUT.  However, historical versions of
lighttpd mod_webdav did provide this (mis-)feature.  Therefore,
provide *deprecated* unsafe partial PUT support for compatibility
with historical lighttpd mod_webdav (prior to mod_webdav rewrite),
but require new config option to enable this compatible behavior:

webdav.opts = ( "deprecated-unsafe-partial-put" => "enable" )

The partial PUT support implemented by historical lighttpd mod_webdav
makes no effort to account for shrinkage or growth of range replaced.
The request body is splat into the *existing* file at the offset
provided which is quite unsafe and can cause corruption in data sent
to concurrent download requests.

Use of this (mis-)feature is discouraged, and support may be removed
in the future, without any further notice.