Fixes and improvements to KIM-1 i/o routines.

Tested on real KIM-1 hardware.

read.s:
- Remove commented out line.
- Remove unused check for bell character.
- Remove echo of newline (hardware always echoes entered characters).
- This fixes gets() and fgets() so they return when CR is entered.

write.s:
- Fix check for adding return after linefeed (failed to work because OUTCHR changes A)
- Remove unused check for bell character.

kim1.inc:
- Add symbol for monitor entry

crt0.s:
- Jump to KIM-1 monitor by address rather than using BRK (which relies on vector being set in RAM)

libsrc/kim1/crt0.s

@@ -40,8 +40,6 @@ _init:    cld                          ; Clear decimal mode
 
           jsr     _main
 
-; Back from main (this is also the _exit entry).  There may be a more elegant way to
-; return to the monitor on the KIM-1, but I don't know it!
-
-_exit:    brk
+; Back from main (this is also the _exit entry).  Jumps to the KIM-1 monitor.
 
+_exit:    jmp   START