moo: usage() > warn("Usage") (#569)

* moo: usage() > warn("Usage")

* Introduce a regular usage() function
* Size of 0 is still not allowed (the puzzle must have 1 or more numbers)

* tweak description text

bin/moo

@@ -16,38 +16,15 @@ use strict;
 
 my ($VERSION) = '1.2';
 
-# Print a usuage message on a unknown option.
-$SIG {__WARN__} = sub {
-    require File::Basename;
-    $0 = File::Basename::basename ($0);
-    if (substr ($_ [0], 0, 14) eq "Unknown option" ||
-        substr ($_ [0], 0,  5) eq "Usage") {
-        warn <<EOF;
-$0 (Perl bin utils) $VERSION
-$0 [size]
-EOF
-        exit 1;
-    }
-    else {
-        warn "$0: @_";
-    }
-};
-
-$SIG {__DIE__} = sub {
-    require File::Basename;
-    $0 = File::Basename::basename ($0);
-    die "$0: @_";
-};
-
-my $size = 4;
-
-warn "Usage" if @ARGV > 1;  # Will exit.
-
-if (@ARGV) {
-    $size = shift;
-    warn "Usage" if !$size || $size =~ /\D/;  # Will exit;
+sub usage {
+    die "usage: moo [size]\n";
 }
 
+my $size = shift;
+$size = 4 unless defined $size;
+usage() if $size !~ m/\A[0-9]+\Z/ or !$size;
+usage() if @ARGV;
+
 print "MOO\n";
 {
     my   @secret_by_value = (0) x 10;
@@ -122,7 +99,7 @@ moo [size]
 
 =head1 DESCRIPTION
 
-I<moo> is a game where the user has to guess a number choosen by
+I<moo> is a game where the user guesses a random number chosen by
 the computer. By default, the computer takes a number of four digits
 (including 0's), but that can be changed by giving I<moo> the number of
 digits to take.  After each guess, the number of B<bull>s and B<cow>s