Make system(NULL) detect whether a command processor is available.

Previously, it just ran an Execute shell call with NULL as the command-line pointer.

The fix is to still call Execute, but with an empty string as the command line, and then check if the call gave an error. If running under plain GS/OS, this gives an error for an unknown system call, whereas both ORCA/APW and GNO shells will return with no error in this case. (Golden Gate doesn't implement the Execute shell call, so it also gives an error and will report no command processor available.)

This fixes issue #25.

stdlib.asm

@@ -973,14 +973,28 @@ system   start
          sta   exComm
          pla
          sta   exComm+2
-         phy                            execute the command
+         ora   exComm
+         sta   empty
+         bne   lb1                      if calling system(NULL)
+         lda   #empty                     use empty command string
+         sta   exComm
+         lda   #^empty
+         sta   exComm+2
+lb1      phy                            execute the command
          phx
          plb
          Execute ex
-         rtl
+         ldy   empty
+         bne   ret                      if doing system(NULL)
+         tya
+         bcs   ret                        error => no command processor
+         inc   a                           (& vice versa)                
+ret      rtl
 
 ex       dc    i'$8000'
 exComm   ds    4
+
+empty    ds    2
          end
 
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