Implement a basic way for read() to block.

FileHandle gets a hasDataAvailableForRead() getter.
If this returns true in sys$read(), the task will block(BlockedRead) + yield.
The fd blocked on is stored in Task::m_fdBlockedOnRead.
The scheduler then looks at the state of that fd during the unblock phase.

This makes "sh" restful. :^)

There's still some problem with the kernel not surviving the colonel task
getting scheduled. I need to figure that out and fix it.

VirtualFileSystem/FileHandle.h

@@ -12,6 +12,8 @@ public:
     Unix::ssize_t read(byte* buffer, Unix::size_t count);
     int stat(Unix::stat*);
 
+    bool hasDataAvailableForRead();
+
     ssize_t get_dir_entries(byte* buffer, Unix::size_t);
 
     ByteBuffer readEntireFile();
@@ -21,6 +23,9 @@ public:
 #ifdef SERENITY
     int fd() const { return m_fd; }
     void setFD(int fd) { m_fd = fd; }
+
+    bool isBlocking() const { return m_isBlocking; }
+    void setBlocking(bool b) { m_isBlocking = b; }
 #endif
 
 private:
@@ -32,6 +37,7 @@ private:
 
 #ifdef SERENITY
     int m_fd { -1 };
+    bool m_isBlocking { true };
 #endif
 };