Ensure that the current drive is selected when exiting a system call.…

… This

avoids issues where the current drive according to the BDOS doesn't match the
current drive according to the BIOS, which can make odd things happen.

src/bdos/filesystem.S

@@ -55,13 +55,20 @@ zproc internal_ENDSYS
     pha
     tya
     pha
+    txa
+    pha
 
     lda old_fcb_drive
     zif_pl
         ldy #FCB_DR
         sta (param), y      ; restore user FCB
     zendif
 
+    lda current_drive       ; reselect the current drive
+    jsr bios_SELDSK
+
+    pla
+    tax
     pla
     tay
     pla
@@ -1818,7 +1825,7 @@ NOINIT
 filesystem_state_start:
 find_first_count:       .byte 0
 active_drive:           .byte 0 ; drive currently being worked on
-old_drive:              .byte 0 ; if the drive has been overridden by the FCB
+old_active_drive:       .byte 0 ; if the drive has been overridden by the FCB
 old_fcb_drive:          .byte 0 ; drive in user FCB on entry
 write_protect_vector:   .word 0
 login_vector:           .word 0