MINOR: sample: provide smp_is_rw() and smp_make_rw()

MINOR: sample: provide smp_is_rw() and smp_make_rw()

At some places, smp_dup() is inappropriately called to ensure a modification
is possible while in fact we only need to ensure the sample may be modified
in place. Let's provide smp_is_rw() to check for this capability and
smp_make_rw() to perform the smp_dup() when it is not the case.

Note that smp_is_rw() will also try to add the trailing zero on strings when
needed if possible, to avoid a useless duplication.