Wording quibbles regarding initdb username

Use disallowed instead of reserved, cannot instead of can not, and
double quotes instead of single quotes.

Also add a test to cover the bug which started this discussion.

Per discussion with Tom.

src/bin/initdb/initdb.c

@@ -3564,7 +3564,7 @@ main(int argc, char *argv[])
 
 	if (strncmp(username, "pg_", 3) == 0)
 	{
-		fprintf(stderr, _("%s: superuser name \"%s\" is reserved; role names can not begin with 'pg_'\n"), progname, username);
+		fprintf(stderr, _("%s: superuser name \"%s\" is disallowed; role names cannot begin with \"pg_\"\n"), progname, username);
 		exit(1);
 	}
 

src/bin/initdb/t/001_initdb.pl

@@ -6,7 +6,7 @@ use strict;
 use warnings;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 14;
+use Test::More tests => 15;
 
 my $tempdir = TestLib::tempdir;
 my $xlogdir = "$tempdir/pgxlog";
@@ -29,6 +29,10 @@ command_fails(
 	[ 'initdb', '-X', 'pgxlog', $datadir ],
 	'relative xlog directory not allowed');
 
+command_fails(
+	[ 'initdb', '-U', 'pg_test', $datadir ],
+	'role names cannot being with "pg_"');
+
 mkdir $datadir;
 command_ok([ 'initdb', '-N', '-T', 'german', '-X', $xlogdir, $datadir ],
 	'successful creation');