Kernel/LibC: Implement basic epoll

This implementation is on top of inodes instead of fds as linux does it.
If I start finding ports/software that relies on epoll allowing
duplicate inodes, I will do what linux does.

I'm probably missing multiple epoll_notify's which may cause hangs but
the system seems to work fine :dd:

kernel/include/kernel/Terminal/PseudoTerminal.h

@@ -29,6 +29,7 @@ namespace Kernel
 		bool can_read_impl() const override { SpinLockGuard _(m_buffer_lock); return m_buffer_size > 0; }
 		bool can_write_impl() const override { SpinLockGuard _(m_buffer_lock); return m_buffer_size < m_buffer->size(); }
 		bool has_error_impl() const override { return false; }
+		bool has_hangup_impl() const override { return !m_slave.valid(); }
 
 		BAN::ErrorOr<long> ioctl_impl(int, void*) override;
 
@@ -63,6 +64,8 @@ namespace Kernel
 	protected:
 		bool putchar_impl(uint8_t ch) override;
 
+		bool has_hangup_impl() const override { return !m_master.valid(); }
+
 		BAN::ErrorOr<long> ioctl_impl(int, void*) override;
 
 	private:

kernel/include/kernel/Terminal/TTY.h

@@ -54,6 +54,7 @@ namespace Kernel
 		virtual bool can_read_impl() const override { return m_output.flush; }
 		virtual bool can_write_impl() const override { return true; }
 		virtual bool has_error_impl() const override { return false; }
+		virtual bool has_hangup_impl() const override { return false; }
 
 	protected:
 		TTY(mode_t mode, uid_t uid, gid_t gid);