modkeep some libraries so PLFORTH can link to them

src/libsrc/apple/conio.pla

@@ -403,4 +403,5 @@ when MACHID & MACHID_MODEL
         break
     otherwise // MACHID_II        puts("Found MACHID_MODEL = $"); putb(MACHID & MACHID_MODEL); putln
 wend
+return modkeep
 done

src/libsrc/apple/dgrlib.pla

@@ -607,4 +607,5 @@ dgrTile:1       = @drawbuff
 dgrTile:6       = @drawbuff+1
 // Put read AUX mem routine in scary location
 memcpy($0100, @auxRead, 9)
+return modkeep
 done

src/libsrc/apple/fileio.pla

@@ -479,4 +479,5 @@ when MACHID & MACHID_MODEL
         break
     otherwise // Apple ][
 wend
+return modkeep
 done

src/libsrc/apple/grlib.pla

@@ -513,4 +513,5 @@ grBLT:1        = @drawbuff
 grBLT:6        = @drawbuff+1
 grTile:1       = @drawbuff
 grTile:6       = @drawbuff+1
+return modkeep
 done

src/libsrc/lines.pla

@@ -503,4 +503,5 @@ _vlineB:6       = @err
 _vlineB:9       = @err
 _vlineB:13      = @err.1
 _vlineB:16      = @err.1
+return modkeep
 done