Fix nlist(3) consumers that either expected our toolchain to prepend an
underscore to symbol names or expected nlist(3) to ignore the mismatch,
as it did until we overhauled it back in May.
While here, also fix cases where the last element in the list had an
empty string instead of NULL as sentinel.
Fixes: 4617a6cb82a6 ("nlist: Handle multiple symbol tables")