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.

Kernel/FileSystem/FileSystem.h

@@ -37,6 +37,7 @@ public:
     virtual bool initialize() = 0;
     virtual const char* class_name() const = 0;
     virtual InodeIdentifier root_inode() const = 0;
+    virtual bool supports_watchers() const { return false; }
 
     bool is_readonly() const { return m_readonly; }