isequal: add --skip for both left and right file

isequal: New option --skip as a shortcut for --skipleft and --skipright
at once.

Use that for the .list-ref files, so they now contain the first line of
the output, too, which is ignored.

test/asm/listing/001-macro-simple-listing.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 001-macro-simple-listing.s
 Current file: 001-macro-simple-listing.s
 

test/asm/listing/002-macro-param-listing.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 002-macro-param-listing.s
 Current file: 002-macro-param-listing.s
 

test/asm/listing/020-asciiz.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 020-asciiz.s
 Current file: 020-asciiz.s
 

test/asm/listing/100-byte.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 100-byte.s
 Current file: 100-byte.s
 

test/asm/listing/101-byt.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 101-byt.s
 Current file: 101-byt.s
 

test/asm/listing/102-word.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 102-word.s
 Current file: 102-word.s
 

test/asm/listing/103-dbyt.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 103-dbyt.s
 Current file: 103-dbyt.s
 

test/asm/listing/104-dword.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 104-dword.s
 Current file: 104-dword.s
 

test/asm/listing/105-faraddr.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 105-faraddr.s
 Current file: 105-faraddr.s
 

test/asm/listing/106-hibytes.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 106-hibytes.s
 Current file: 106-hibytes.s
 

test/asm/listing/107-lobytes.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 107-lobytes.s
 Current file: 107-lobytes.s
 

test/asm/listing/Makefile

@@ -106,7 +106,7 @@ ifneq ($(wildcard $1.list-ref),)
 #	we have a reference file, compare that, too
 
 #	remove first line which contains a version number
-	$(ISEQUAL) --skipright=1 $1.list-ref $$(@:.bin=.lst)
+	$(ISEQUAL) --skip=1 $1.list-ref $$(@:.bin=.lst)
 endif
 
 endef # LISTING_template

test/isequal.c

@@ -36,9 +36,11 @@ int handleparameter(int argc, char *argv[])
     static const char opt_empty[]     = "--empty";
     static const char opt_skipleft[]  = "--skipleft";
     static const char opt_skipright[] = "--skipright";
+    static const char opt_skip[]      = "--skip";
 
     static const char len_skipleft    = sizeof opt_skipleft - 1;
     static const char len_skipright   = sizeof opt_skipright - 1;
+    static const char len_skip        = sizeof opt_skip - 1;
 
     int argindex = 1;
 
@@ -81,6 +83,16 @@ int handleparameter(int argc, char *argv[])
                 fprintf(stderr, "%s: you must specify the number of lines\n", opt_skipright);
             }
         }
+        else if (strncmp(argv[argindex], opt_skip, len_skip) == 0) {
+            if (binary || empty) {
+                fprintf(stderr, "%s cannot go with other options.\n", opt_skipright);
+                exit(1);
+            }
+            skiplines_left = skiplines_right = handleargparameter(len_skip, argv[argindex]);
+            if (skiplines_left < 0) {
+                fprintf(stderr, "%s: you must specify the number of lines\n", opt_skip);
+            }
+        }
         ++argindex;
     }