Make seek work on /dev/null (#22886)

Seeking `/dev/null` should work and do nothing instead of returning
`EPIPE`. This fixes it.

Author: hoodmane

src/library_fs.js

@@ -1347,6 +1347,7 @@ FS.staticInit();
       FS.registerDevice(FS.makedev(1, 3), {
         read: () => 0,
         write: (stream, buffer, offset, length, pos) => length,
+        llseek: () => 0,
       });
       FS.mkdev('/dev/null', FS.makedev(1, 3));
       // setup /dev/tty and /dev/tty1

test/test_files.c

@@ -162,6 +162,8 @@ void test_tempfiles() {
   char data[5] = { 10, 30, 20, 11, 88 };
   FILE *n = fopen("/dev/null", "w");
   printf("5 bytes to dev/null: %zu\n", fwrite(data, 1, 5, n));
+  int res = fseek(n, 10, SEEK_CUR);
+  printf("seek /dev/null: %d\n", res);
   fclose(n);
 
   // Test file creation with O_TRUNC (regression test for #16784)

test/test_files.out

@@ -12,4 +12,5 @@ seeked=ata.
 seeked=ta.
 fscanfed: 10 - hello
 5 bytes to dev/null: 5
+seek /dev/null: 0
 ok.