LwIP: Rework code to use all of libcoap capabilities

[mrdeep1]

Update examples/lwip directory to make it more standalone.

Available directories under examples/lwip/

config/ Contains LwiP + CoAP configuration files
libcoap/ Link to the libcoap tree
lwip/ The LwIP tree
lwip-contrib/ The LwIP contrib tree (obsoleted in LwIP master branch)
src/ Local modifications to LwIP
tinydtls/ Link to the TinyDTLS tree

New files to contain the headers for the appropriate build types, with most
of the #include removed from the other *.[ch] files.

App visibility (selected by build type)
include/coap3/coap_include_contiki.h
include/coap3/coap_include_lwip.h
include/coap3/coap_include_posix.h
include/coap3/coap_include_windows.h
library visibility (Can have tests for include files etc.)
include/coap3/coap_include_contiki_internal.h
include/coap3/coap_include_lwip_internal.h
include/coap3/coap_include_posix_internal.h
include/coap3/coap_include_windows_internal.h

mem.[ch] renamed to coap_mem.[ch] to prevent build name clashes.

Changed NO_SYS to 0, and so now accept() is renamed to lwip_accept(), socket()
is renamed to lwip_socket() etc.

Add in TinyDTLS support for packet encryption.

Removed a lot of the WITH_LWIP protected code as it is now redundant.

Removed LwIP specific files no longer in use.

Uses the tap interface to interface with Linux, each new application will
use the next available tap interface. tap interfaces can be pre-configured
and used.

Addresses issues #18, #59, #61, #63, #334, #403, #520, #526, #527, #528, #529 and #591

[mrdeep1]

Likely to be superseded by #926.

[mrdeep1]

Superseded by #939