Add error reporting (via errno) to getauxval().

[BZ 15846] As discussed in the recent thread on my $EXEC_ORIGIN patch
and in BZ 15846, getauxval() presently has no unambiguous way of
reporting an error condition.  It currently returns zero on error, but
this may also be a valid result for some auxv entries.  As there is no
clear invalid result for all current and future auxv entries, this patch
sets errno (following a suggestion in the BZ entry).

This version of the patch also adds documentation and tests for the
value-not-found conditions in getauxval().