There is no reason for this variable to be external. It is used in init() and
was used in done(), but the unlink() in done() is useless: it was already called
on init() so the file was already unlinked.

Provide MDP sockets instead of using a global one.

I removed the definition of mdp_client_socket, but an external declaration was
remaining.

lookup_send_request() must use the socket of the caller, not create a new
one (detected by dnaprotocol test script).

Else, once the buffer is full, we get errno=11

ReleaseByteArrayElements last parameter is mode. Its value is:
 - 0: copy back the content and free the buffer;
 - JNI_COMMIT: copy back the content and *do not* free the buffer;
 - JNI_ABORT: free the buffer without copying back the possible changes.

 We definitely want 0.

 With JNI_COMMIT, after receiving more than ~1024 packets, it crashes:
   ReferenceTable overflow (max=1024)

Conflicts:
	commandline.c
	mdp_client.c
	serval.h
	sourcefiles.mk

(unused, signed/unsigned variables...)

Using fd+1 is the right way to use select()

Avoid to keep a lot of useless files like:
  var/serval-node/mdp-client-XXXX-XXXXXXXX.socket

If serval does not close properly, socket files are kept in
/data/data/org.servalproject/var/serval-node. Therefore, we need to clean up
when servald starts.

Conflicts:
	mdp_client.c

Makes any blocking recv exit

Blocking recv calls return a special return code (-2) for notifying the socket
is closed (this information is useful for MDP socket users).

This implementation is unsatisfactory: return code should instead be consistent
with recvfrom(), that is always returning the length of received data (0 for
socket closed).

Conflicts:
	commandline.c
	mdp_client.c
	serval.h

Avoid to open/close it if something goes wrong early.

Conflicts:
	commandline.c
	mdp_client.c
	serval.h

Conflicts:
	commandline.c
	mdp_client.c
	serval.h