Split and remove lttng-share header file

This header is split into two files: defaults.h and macros.h.

A third header is introduce here which is common.h including runas.h and
macros.h which are part of the common library. Since the defaults.h
contains only defaults values, it seems ok to keep it seperate from the
common library having only macros and function calls.

Signed-off-by: David Goulet <dgoulet@efficios.com>