Add hash_free_all() to factor common code of iterating over all
members of a hash to free them with a single nasm_free().
Split strtbl_find() into strtbl_find() and strtbl_add(). It is very
unlikely that the same call site will want to have both of these
functionalities, and in the end the code for the two functions are
surprisingly different.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>