tt/mini_isam: add new isam_prototypes.h include file and fix up problems

There were a variety of potential 64b problems due to the complete
lack of prototype declarations.  So, we add a file, isam_prototypes.h,
generated mostly by the 'cproto' tool.

We also fixed up some errors that having proper prototypes pointed
out, mainly in passing ints where pointers were expected and vice
versa.  The iserase() function was supposed to accept a char *, but
was defined as only accepting a char.  Fun stuff like that.

cde/lib/tt/mini_isam/isvarrec.c

@@ -44,7 +44,7 @@ int  _vl_deleted();			     /* 0/1 returns 1 if record is deleted */
 static void remove_from_chain2();	     /* used by _vlrec_wrrec() */
 long _istail_insert();
 static void _istail_delete();
-static _istail_read();
+static int _istail_read();
 
 /*
  * _vlrec_write(fcb, record, recnum, reclen)
@@ -501,7 +501,7 @@ Static void _istail_delete(Fcb *fcb, long offset)
 
 /* Read tail from .var file */
 
-Static _istail_read(Fcb *fcb, long offset, char *buffer)
+Static int _istail_read(Fcb *fcb, long offset, char *buffer)
 {
     char		frameheadbuf [2 * SHORTSIZE];
     int			taillen;