iscan*: disable gimp2 plugin, fix nonstandard sscanf, restrict according to license

[yoshiyoshyosh]

Testing the changes

• I tested the changes in this PR: YES
  • I have a perfection V600 photo and when combined with my other PR for adding that plugin, it works perfectly.

Local build testing

• my native: x86_64-glibc

The Epson End User Software License Agreement does not allow redistribution, so these packages need to be restricted. Only the gui frontend and plugin use the nonfree license--sane-epkowa is gpl and fine to be in main repos.

The sscanf patch is to fix incorrect C99 based on the AUR patch. Needed for every nonfree scanner to be discovered by SANE.

[yoshiyoshyosh]

note: the absolute paths as arguments to iscan-registry in the INSTALL/REMOVE scripts are read at runtime, not at install time. if they were relative, they would be read relative to the cwd that SANE is called from, which obviously doesn't work out. I don't think #2149 actually tested against an actual scanner when removing the absolute paths.