common: prevent using memset on non-POD types

common: prevent using memset on non-POD types

While converting some code to use C++ constructs, it can be easy to
forget to change some spot that uses memset to initialize or move the
object. Add a templated deleted declaration to prevent using memset on
types that aren't POD.

For example, if I make lttng_ust_event non-POD, in
src/bin/lttng-sessiond/trace-ust.h, I get this error:

      CXX      save.lo
    /home/simark/src/lttng-tools/src/bin/lttng-sessiond/save.cpp: In function ‘int save_agent_events(config_writer*, agent*)’:
    /home/simark/src/lttng-tools/src/bin/lttng-sessiond/save.cpp:1246:23: error: use of deleted function ‘void* memset(T*, int, size_t) [with T = ltt_ust_event; <template-parameter-1-2> = void; size_t = long unsigned int]’
     1246 |                 memset(&fake_event, 0, sizeof(fake_event));
          |                 ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In file included from /home/simark/src/lttng-tools/src/common/defaults.h:14,
                     from /home/simark/src/lttng-tools/src/bin/lttng-sessiond/save.cpp:15:
    /home/simark/src/lttng-tools/src/common/macros.h:128:7: note: declared here
      128 | void *memset(T *s, int c, size_t n) = delete;
          |       ^~~~~~

Note: I tried applying this to memcpy as well, but Clang gave me some
troubles with its -Waddress-of-packed-member diagnostic, so I gave up.

Change-Id: Id55735db15901c6fc5d58e9b6b6b689733302398
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>