Kernel+FileManager: Disallow watch_file() in unsupported file systems

Currently only Ext2FS and TmpFS supports InodeWatchers. We now fail
with ENOTSUPP if watch_file() is called on e.g ProcFS.

This fixes an issue with FileManager chewing up all the CPU when /proc
was opened. Watchers don't keep the watched Inode open, and when they
close, the watcher FD will EOF.

Since nothing else kept /proc open in FileManager, the watchers created
for it would EOF immediately, causing a refresh over and over.

Fixes #879.

Libraries/LibC/string.cpp

@@ -406,6 +406,8 @@ const char* const sys_errlist[] = {
     "Wrong protocol type",
     "Operation in progress",
     "No such thread",
+    "Protocol error",
+    "Not supported",
     "The highest errno +1 :^)",
 };