html: remove redundant htmlfd variable

This is never changed from STDOUT_FILENO, so just use that value
directly.

Signed-off-by: John Keeping <john@keeping.me.uk>
ui-snapshot: set unused cgit_filter fields to zero

By switching the assignment of fields in the cgit_filter structure to
use designated initializers, the compiler will initialize all other
fields to their default value.  This will be needed when we add the
extra_args field in the next patch.

Signed-off-by: John Keeping <john@keeping.me.uk>
filter: pass extra arguments via cgit_open_filter

This avoids poking into the filter data structure at various points in
the code.  We rely on the fact that the number of arguments is fixed
based on the filter type (set in cgit_new_filter) and that the call
sites all know which filter type they're using.

Signed-off-by: John Keeping <john@keeping.me.uk>