plug-ins: change file-[gbr/gih/pat]-save to export

Followup to c92cf7e8

This changes file-[gbr/gih/pat]-save to *-export, it was covering up other scripts bug I intended to look at 😆

Example of error on run without this fix:

scriptfu-Message: 09:20:25.668: Error while executing script-fu-make-brush-rectangular:

Error: eval: unbound variable: file-gbr-save 

(I have only touched the pure proc name, there are also some uses ending with *-save-internal, and *-save-last used as part of a parasite name.)