Slightly modernize ext_skel and sekelton/

- .svnignore ->.gitignore
- Add .gitignore entries for DSO stand-alone builds stuff
- Docs don't use CVS but SVN
- reorder extname.c file so it needs less forward declarations
- take forward declarations out of php_extname.h
- Drop #if for 12 years old PHP version compatibility
Drop PHP 4 related README
Remove PHP3 conversion scripts