Moved two errno helper functions into separate files.

Only the function that actually is needed will be linked.  It is very useful for targets that do not have file-system I/O.

libsrc/common/_directerrno.s

@@ -0,0 +1,27 @@
+;
+; 2003-08-12, Ullrich von Bassewitz
+; 2015-09-24, Greg King
+;
+; Helper function for several high-level file functions.
+;
+
+        .include        "errno.inc"
+
+        .macpack        cpu
+
+; ----------------------------------------------------------------------------
+; int __fastcall__ _directerrno (unsigned char code);
+; /* Set errno to a specific error code, clear _oserror, and return -1. Used
+; ** by the library.
+; */
+
+__directerrno:
+        jsr     __seterrno              ; Set errno (returns with .A = 0)
+        sta     __oserror               ; Clear __oserror
+.if (.cpu .bitand CPU_ISET_65SC02)
+        dec     a
+.else
+        lda     #$FF                    ; Return -1
+.endif
+        tax
+        rts