To avoid inconsitstant definition of "bool", the definition of
mutex_type is moved into its own header file. This way, Socket.h does
not need to include Thread.h.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

* develop: (21 commits)
  Add OpenSSL version check for X509_check_host
  Try to update OpenSSL on Ubuntu$
  Add TLS config for client and server for hostname verification #420
  Add TLS hostname check and CApath #420 #418
  Add tests for wrong protocol name #294
  Check for bad protocol name #294
  Check for SSLOptions structure when SSL protocol prefix is used #334
  Update CONTRIBUTING.md CLA info
  Remove reference to exit() function #342
  Reinstate tests for appveyor
  Enclose references to OsWrapper in #if defines
  Squashed commit of the following:
  Package AppVeyor artifacts attempt 1
  Create /usr/local/include if it doesn't exist #275
  Remove 373 test from Makefile... add the test source for posterity #416
  Add doc for token in ResponseOptions #411
  Add missing string.h include to SSLSocket.c
  Try AppVeyor config again
  Update AppVeyor config for latest test broker updates
  Move ListRemove after where qe is used #339
  ...

# Conflicts:
#	src/SSLSocket.c
#	test/test_issue373.c
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

As suggested in https://github.com/eclipse/paho.mqtt.c/issues/385#issuecomment-372299490
the socket_mutex is released during the call of select(),
to avoid a performance penalty.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

To reduce the performance penalty of commit 779ca42a
Ian has recommended to release the socket_mutex during the call of select.
See https://github.com/eclipse/paho.mqtt.c/issues/385#issuecomment-372299490

As a preparation for this, the locking of the socket_mutex is moved inside
the Socket_getReadySocket() function.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>

Add missing string.h include to SSLSocket.c