Remove extern int references to errno and rely on the #include

Fixes compilation against glibc-2.3+.

[Description and fix to error.3 by Ismael Luceno <ismael@iodev.co.uk>]
Import qmail-ldap 19980906
Import qmail-ldap 19981025
Import qmail-ldap 19981114
Import qmail-ldap 19990501
Import qmail-ldap 19990525
Import qmail-ldap 19990805
Import qmail-ldap 19991007
Import qmail-ldap 20000301-preview
Import qmail-ldap 20000325
Import qmail-ldap 20000401
Import qmail-ldap 20000501
Import qmail-ldap 20000601
Import qmail-ldap 20000701
Import qmail-ldap 20001201
Import qmail-ldap 20010101
Import qmail-ldap 20010201
Import qmail-ldap 20010301
Import qmail-ldap 20010501
Import qmail-ldap 20011001a