Support for socket-descriptor marshalling

This patch adds prinicipal support for transmitting socket descriptors
as RPC payload. Socket descriptors are handled by the linux-specific
implementation of the capability marshalling and unmarshalling functions
in 'ipc.h'. The 'Message' type in 'src/platform/linux_socket.h' has been
extended to carry multiple descriptors in a single message.

Unfortuately, we hit a problem (and potential show stopper) here:

  lx_sendmsg failed with -109 in lx_call()

The error code corresponds to ETOOMANYREFS. There is only one place in
the Linux kernel where this error code is used (net/unix/af_unix.c).
The code for 'unix_attach_fds()' suggests that there is a limit with
regard to the maximum number of references for a given Unix domain
socket. When the error occurs, core and init are running. The socket
of core's server entrypoint is present in the '/proc/pid/fd' of those
processes 8 times. The error occurs when core tries to perform an
RPC to the entrypoint to perform 'Ram_session::transfer_quota()'
(base/include/base/child.h at line 248).

base-linux/include/base/ipc_msgbuf.h

@@ -21,10 +21,30 @@ namespace Genode {
 	 */
 	class Msgbuf_base
 	{
+		public:
+
+			enum { MAX_CAPS_PER_MSG = 8 };
+
 		protected:
 
+			/*
+			 * Capabilities (file descriptors) to be transferred
+			 */
+			int            _caps[MAX_CAPS_PER_MSG];
+			Genode::size_t _used_caps;
+			Genode::size_t _read_cap_index;
+
+			/**
+			 * Maximum size of plain-data message payload
+			 */
 			Genode::size_t _size;
-			char           _msg_start[];  /* symbol marks start of message buffer data */
+
+			/**
+			 * Actual size of plain-data message payload
+			 */
+			Genode::size_t _used_size;
+
+			char _msg_start[];  /* symbol marks start of message buffer data */
 
 			/*
 			 * No member variables are allowed beyond this point!
@@ -34,6 +54,8 @@ namespace Genode {
 
 			char buf[];
 
+			Msgbuf_base() { reset_caps(); }
+
 			/**
 			 * Return size of message buffer
 			 */
@@ -43,6 +65,36 @@ namespace Genode {
 			 * Return address of message buffer
 			 */
 			inline void *addr() { return &_msg_start[0]; };
+
+			void reset_caps() { _used_caps = 0; _read_cap_index = 0; }
+
+			bool append_cap(int cap)
+			{
+				if (_used_caps == MAX_CAPS_PER_MSG)
+					return false;
+
+				_caps[_used_caps++] = cap;
+				return true;
+			}
+
+			int read_cap()
+			{
+				if (_read_cap_index == _used_caps)
+					return -1;
+
+				return _caps[_read_cap_index++];
+			}
+
+			size_t used_caps() const { return _used_caps; }
+
+			int cap(unsigned index) const
+			{
+				return index < _used_caps ? _caps[index] : -1;
+			}
+
+			size_t used_size() const { return _used_size; }
+
+			void used_size(size_t used_size) { _used_size = used_size; }
 	};