LibC: Improve error handling for unimplemented functions

- getpriority()/setpriority(): Set errno to ENOSYS before returning -1
- chroot(): Set errno to ENOSYS before returning -1
- nice(): Set errno to ENOSYS and return -1 (was incorrectly returning incr)
- fnmatch(): Return FNM_NOMATCH instead of 0 (match) for unimplemented stub

These functions now properly indicate they are not implemented by setting
errno to ENOSYS, following POSIX conventions. The fnmatch() change prevents
false positive matches when the function is called before implementation.

Author: po-nuvai

Userland/Libraries/LibC/fnmatch.cpp

@@ -4,11 +4,12 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/Format.h>
 #include <fnmatch.h>
 
-int fnmatch(char const*, char const*, int)
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/fnmatch.html
+int fnmatch([[maybe_unused]] char const* pattern, [[maybe_unused]] char const* string, [[maybe_unused]] int flags)
 {
-    dbgln("FIXME: Implement fnmatch()");
-    return 0;
+    // FIXME: Implement fnmatch()
+    // Returning FNM_NOMATCH is safer than returning 0 (match) for unimplemented function
+    return FNM_NOMATCH;
 }

Userland/Libraries/LibC/priority.cpp

@@ -4,20 +4,24 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/Format.h>
+#include <errno.h>
 #include <sys/resource.h>
 
 extern "C" {
 
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/getpriority.html
 int getpriority([[maybe_unused]] int which, [[maybe_unused]] id_t who)
 {
-    dbgln("FIXME: Implement getpriority()");
+    // FIXME: Implement getpriority()
+    errno = ENOSYS;
     return -1;
 }
 
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/setpriority.html
 int setpriority([[maybe_unused]] int which, [[maybe_unused]] id_t who, [[maybe_unused]] int value)
 {
-    dbgln("FIXME: Implement setpriority()");
+    // FIXME: Implement setpriority()
+    errno = ENOSYS;
     return -1;
 }
 }

Userland/Libraries/LibC/unistd.cpp

@@ -1110,9 +1110,10 @@ int pause()
 }
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/chroot.html
-int chroot(char const* path)
+int chroot([[maybe_unused]] char const* path)
 {
-    dbgln("FIXME: chroot(\"{}\")", path);
+    // FIXME: Implement chroot()
+    errno = ENOSYS;
     return -1;
 }
 
@@ -1125,10 +1126,11 @@ int getdtablesize()
 }
 
 // https://pubs.opengroup.org/onlinepubs/007904975/functions/nice.html
-int nice(int incr)
+int nice([[maybe_unused]] int incr)
 {
-    dbgln("FIXME: nice was called with: {}, not implemented", incr);
-    return incr;
+    // FIXME: Implement nice()
+    errno = ENOSYS;
+    return -1;
 }
 
 int brk(void* addr)