check if the file descriptor can be stored in the select() file descriptor set (r1781 from 0.8, r1782 from 0.7)

git-svn-id: http://arthurdejong.org/svn/nss-pam-ldapd/nss-pam-ldapd-0.7.15+squeeze@1926 ef36b2f9-881f-0410-afb5-c4e39611909c