Fixed "cannot fork" error in Windows Web PVR Manager (with caveats)

The WPM must call waitpid() on Windows even though zombie child processes
are automatically reaped. This resets internal Perl data structures and
permits the WPM to fork more than 64 child processes, and thus enables
the pvr list to auto-run and the cache to auto-refresh for extended
periods of time.

However, waitpid() must be called in the main server loop on Windows (no
SIGCHLD generated). This means there is the potential for errors when an
auto-run pvr request comes at the same time as a request generated by
another WPM task in another window/tab. Windows users will need to avoid
that situation. Warning messages have been added to the auto-loading pages
(Windows only).

This does not affect Unix/OS X users or CGI deployments of the WPM.

Closes #268

get_iplayer.cgi

@@ -306,6 +306,7 @@ my @nosearch_params = qw/ /;
 ### Perl CGI Web Server ###
 use Socket;
 use IO::Socket;
+use POSIX ":sys_wait_h";
 my $IGNOREEXIT = 0;
 # If the port number is specified then run embedded web server
 if ( $opt_cmdline->{port} > 0 ) {
@@ -332,6 +333,10 @@ if ( $opt_cmdline->{port} > 0 ) {
 			# Parent
 			if ( $procid ) {
 				close $client;
+				# must call waitpid() on Windows
+				if ( IS_WIN32 ) {
+					while ( abs(waitpid(-1, WNOHANG)) > 1 ) {}
+				}
 				next;
 			}
 			# Child
@@ -739,6 +744,9 @@ sub run_cgi {
 
 sub pvr_run {
 	print $fh "<strong><p>The PVR will auto-run every $opt->{AUTOPVRRUN}->{current} hour(s) if you leave this page open</p></strong>" if $opt->{AUTOPVRRUN}->{current};
+	if ( IS_WIN32 ) {
+		print $fh "<strong><p>Windows users: You may encounter errors if you perform other tasks in the Web PVR Manager while this page is reloading</p></strong>" if $opt->{AUTOPVRRUN}->{current};
+	}
 	print $se "INFO: Starting PVR Run\n";
 	my @cmd = (
 		$opt_cmdline->{getiplayer},
@@ -2740,6 +2748,9 @@ sub refresh {
 	my $typelist = join(",", $cgi->param( 'PROGTYPES' )) || 'tv';
 	my $refreshfuture = $cgi->param( 'REFRESHFUTURE' ) || 0;
 	print $fh "<strong><p>The cache will auto-refresh every $opt->{AUTOWEBREFRESH}->{current} hour(s) if you leave this page open</p></strong>" if $opt->{AUTOWEBREFRESH}->{current};
+	if ( IS_WIN32 ) {
+		print $fh "<strong><p>Windows users: You may encounter errors if you perform other tasks in the Web PVR Manager while this page is reloading</p></strong>" if $opt->{AUTOWEBREFRESH}->{current};
+	}
 	print $se "INFO: Refreshing\n";
 	my @cmd = (
 		$opt_cmdline->{getiplayer},