sysconf(NPROCESSORS_CONF) is broken on musl and they've been arguing …

…about

it for years (https://www.openwall.com/lists/musl/2022/07/27/5) so just
inline the readdir glibc and bionic are both doing under the covers.

In theory we could also readdir for the online case, but the syscall works
when sysfs isn't mounted. (And while I'm there: never return 0.)

toys/other/taskset.c

@@ -114,15 +114,26 @@ void taskset_main(void)
 void nproc_main(void)
 {
   unsigned i, j, nproc = 0;
+  DIR *dd;
 
   // This can only detect 32768 processors. Call getaffinity and count bits.
-  if (!toys.optflags && -1!=sched_getaffinity(getpid(), 4096, toybuf)) {
+  if (!toys.optflags && -1!=sched_getaffinity(getpid(), 4096, toybuf))
     for (i = 0; i<4096; i++)
       if (toybuf[i]) for (j=0; j<8; j++) if (toybuf[i]&(1<<j)) nproc++;
-  }
 
-  // If getaffinity failed or --all, count cpu entries in proc
-  if (!nproc) nproc = sysconf(_SC_NPROCESSORS_CONF);
+  // If getaffinity failed or --all, count cpu entries in sysfs
+  // (/proc/cpuinfo filters out hot-unplugged CPUs, sysfs doesn't)
+  if (!nproc && (dd = opendir("/sys/devices/system/cpu"))) {
+    struct dirent *de;
+    char *ss;
+
+    while ((de = readdir(dd))) {
+      if (memcmp(de->d_name, "cpu", 3)) continue;
+      for (ss = de->d_name+3; isdigit(*ss); ss++);
+      if (!*ss) nproc++;
+    }
+    closedir(dd);
+  }
 
-  xprintf("%u\n", nproc);
+  xprintf("%u\n", nproc ? : 1);
 }