Force dumplease to parse stdin

Checking if data exists in the stdin buffer via an ioctl is
unreliable. Allow a user to pass '-' to force stdin parsing.
This provides a fix that will allow "no interfaces" to still
mean "dump all interfaces".

Fixes #285

src/dhcpcd.c

@@ -29,7 +29,6 @@
 static const char dhcpcd_copyright[] = "Copyright (c) 2006-2023 Roy Marples";
 
 #include <sys/file.h>
-#include <sys/ioctl.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/time.h>
@@ -2278,8 +2277,10 @@ main(int argc, char **argv, char **envp)
 
 #ifndef SMALL
 	if (ctx.options & DHCPCD_DUMPLEASE &&
-	    ioctl(fileno(stdin), FIONREAD, &i, sizeof(i)) == 0 &&
-	    i > 0)
+	    i > 0 &&
+	    ctx.ifc == 1 &&
+	    strlen(ctx.ifv[0]) == 1 &&
+	    ctx.ifv[0][0] == '-')
 	{
 		ctx.options |= DHCPCD_FORKED; /* pretend child process */
 #ifdef PRIVSEP