env: '--' is valid option terminator

* Test case repeated for GNU and OpenBSD env

%env -i -- O=o printenv
O=o

* This code doesn't use getopt so add '--' manually for now
* Remove ENVIRONMENT section of the manual which is full of vermin lies

bin/env

@@ -34,6 +34,8 @@ while ( @ARGV && $ARGV[0] =~ /^-/ ) {
 			exit 2;
 		}
 		delete $ENV{$val};
+	} elsif ($arg eq '--') {
+		last;
 	} else {
 		warn "$Program: invalid option -- $arg\n";
 		exit 2;
@@ -94,10 +96,6 @@ The value must not include the '=' character.
 
 =back
 
-=head1 ENVIRONMENT
-
-The working of I<env> is not influenced by any environment variables.
-
 =head1 DIAGNOSTICS
 
 If the command is invoked, the exit status of I<env> will be the exit