Move the prototype for show_content_aux() to a common header file; it

Move the prototype for show_content_aux() to a common header file; it
got an extra argument recently but none of the other users of it
noticed.

When fixing this I discovered that the usage of show_content_aux()
by mhstoresbr.c was wrong; the prototype had an extra argument in
the middle of it.  Apparently this has been broken for 15 years!