Changes

session.h: Move inclusion of string.h into .c files

string.h is required for memcmp() that is used in session.c and
dtls_debug.c. Therefore, the header file needs to be included
only in these two files.

Change-Id: I11db1c37a7cc13d8fa2826f6d3e38d3765f4a4ec

tests: Fix compiler clash over index usage in test suites

Some compilers complain that index is already defined in <strings.h>.
Replace index with lindex.

Signed-off-by: Jon Shallow <supjps-libcoap@jpshallow.com>

dtls_debug: mock dsrv_log() when NDEBUG is set

Setting NDEBUG already replaces dtls_dsrv_hexdump_log(), dtls_dsrv_log_addr()
etc with dummy implementations.

This extends this to also replace dsrv_log() and dtls_{get,set}_log_level()
with a no-op implementation.