Move strerror_r to compat directory

Move strerror_r to compat directory

The usage of strerror_r is platform dependent.
Add new function compat_strerror_r to
include/babeltrace/compat/string.h,
and hide the platform-dependent usage there.
Use the aforementioned header file instead of standard string.h where
necessary.

[ Updates by Mathieu Desnoyers: fix coding style, fix possible buffer
  overflow by using strncpy rather than strcpy. ]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>