Use perl's $/ more idiomatically

This replaces a few occurrences of ugly code with a more clean and
idiomatic usage. The problem was highlighted by perlcritic, but we're
not enforcing the policy that led to the discovery.

Discussion: https://postgr.es/m/20200412074245.GB623763@rfd.leadboat.com

src/tools/msvc/Install.pm

@@ -762,13 +762,10 @@ sub read_file
 {
 	my $filename = shift;
 	my $F;
-	my $t = $/;
-
-	undef $/;
+	local $/ = undef;
 	open($F, '<', $filename) || die "Could not open file $filename\n";
 	my $txt = <$F>;
 	close($F);
-	$/ = $t;
 
 	return $txt;
 }

src/tools/msvc/Project.pm

@@ -420,13 +420,10 @@ sub read_file
 {
 	my $filename = shift;
 	my $F;
-	my $t = $/;
-
-	undef $/;
+	local $/ = undef;
 	open($F, '<', $filename) || croak "Could not open file $filename\n";
 	my $txt = <$F>;
 	close($F);
-	$/ = $t;
 
 	return $txt;
 }
@@ -435,15 +432,12 @@ sub read_makefile
 {
 	my $reldir = shift;
 	my $F;
-	my $t = $/;
-
-	undef $/;
+	local $/ = undef;
 	open($F, '<', "$reldir/GNUmakefile")
 	  || open($F, '<', "$reldir/Makefile")
 	  || confess "Could not open $reldir/Makefile\n";
 	my $txt = <$F>;
 	close($F);
-	$/ = $t;
 
 	return $txt;
 }

src/tools/win32tzlist.pl

@@ -60,12 +60,13 @@ $basekey->Close();
 # Fetch all timezones currently in the file
 #
 my @file_zones;
+my $pgtz;
 open(my $tzfh, '<', $tzfile) or die "Could not open $tzfile!\n";
-my $t = $/;
-undef $/;
-my $pgtz = <$tzfh>;
+{
+	local $/ = undef;
+	$pgtz = <$tzfh>;
+}
 close($tzfh);
-$/ = $t;
 
 # Attempt to locate and extract the complete win32_tzmap struct
 $pgtz =~ /win32_tzmap\[\] =\s+{\s+\/\*[^\/]+\*\/\s+(.+?)};/gs