* posix.c (scm_convert_exec_args), dynl.c

	* posix.c (scm_convert_exec_args), dynl.c
	(scm_make_argv_from_stringlist): static procs: 1) renamed both to
	allocate_string_pointers.  2) simplified: don't reallocate the
	strings, just make an array of pointers 3) avoid memory leaks on
	error 4) let the procedure report errors in its own name.
	Consequences: 1) the procedures now assume that SCM strings are
	nul-terminated, which should always be the case.  2) Since strings
	are not reallocated, it's now possible for strings passed to
	dynamic-args-call to be mutated.