split_args(): Do not insist on checking the return value.

split_args(): Do not insist on checking the return value.

Currently, split_args() of string.c has the __must_check attribute
which instructs gcc to warn whenever the return value of this function
is ignored by the caller.

However, since the returned array is NULL terminated anyway, there
are situations the return value my safely be ignored, for example if
the returned array is passed to execvp(), which does not receive a
length argument.

This patch removes the __must_check attribute and fixes a "set but
not used" warning on newer gcc versions.