libsrc/*/kplot.s: Use cbm_kernal.inc symbols, not hardcoded addrs

On C64, VIC-20 and Plus/4, the conio library PLOT routine uses direct
calls into the Kernal, including the Kernal PLOT routine that we're
replacing. These were previously hardcoded addresses; we change these
to use the symbols for those routines defined in cbm_kernal.inc. (This
changes no functionality.)

To do this, we need to import cbm_kernal.inc in a namespace so we
don't have a collision between the PLOT that we're defining and the
Kernal definition.

We also add a UPDCRAMPTR symbol (used by kplot for VIC-20 and C64) to
the direct entry kernal routines in in cbm_kernal.inc, and expand the
comments describing what the "direct entry" Kernal routines are.

<greg.king5@verizon.net> (GitHub: greg-king5) came up with this idea
and did initial testing of it.

This has been tested on VICE xvic, x64 and xplus4 emulators with a
program that does a cputs() call (github.com/0cjs/vic20cc65) to
confirm that it works the same way after as it did before.

asminc/cbm_kernal.inc

@@ -100,14 +100,26 @@ UDTIM          := $FFEA
 
 ; ---------------------------------------------------------------------------
 ; Kernal routines, direct entries
+;
+; Unlike the above, these are not standard functions with entries in the jump
+; table. They do not exist in all Kernals, and where they do the entry point is
+; specific to that particular machine and possibly even Kernal version.
+;
+; This list is not comprehensive: missing items for particular machines
+; should be added as needed.
+;
+; UPDCRAMPTR: Updates the color RAM pointer to match the screen RAM pointer.
+;
 
 .if .def(__VIC20__)
   CLRSCR       := $E55F
   KBDREAD      := $E5CF
+  UPDCRAMPTR   := $EAB2
 .elseif .def(__C64__)
   CLRSCR       := $E544
   KBDREAD      := $E5B4
   NMIEXIT      := $FEBC
+  UPDCRAMPTR   := $EA24
 .elseif .def(__C128__)
   CLRSCR       := $C142
   KBDREAD      := $C006