elf_aux_info(3): return correct errors

[mhorne]

Summary

Some entries return ENOENT when the provided buffer is too small. Change
them to match the behaviour of the rest and return EINVAL in these
cases.

While here, fix a typo in the manpage: the function returns ENOENT when
a non-existent entry is requested, not EINVAL.

[mhorne]

Update after applying the buflen patch later in the series.

[markj]

Approved.