Create new plugin file script-fu-server-plugin.c
with code extracted from script-fu.c, which is an omnibus plugin
implementing PDB proc extension-script-fu and other PDB procs.
Why:
1. extension-script-fu is smaller and doesn't doesn't link to socket libraries.
(GIMP always starts extension-script-fu and it stays running.)
2. packagers/admins can omit script-fu-server executable from an installation,
if they think letting users serve net ports is not secure.
3. crashing script-fu-server does not crash extension-script-fu,
which requires restart of GIMP
The changes are mostly a simple refactor, extracting code.
No functional change apparent to users.
Low risk of introduced bugs.
Extremely few users use script-fu-server anyway.
Added some logging.
While at it, use G_DECLARE_FINAL_TYPE
