Fixed a regression in the usertab file not accepting IPv6 addresses any more.

Fixed a regression in the usertab file not accepting IPv6 addresses any more.
Reintroduced in user_name the buffer size check that was present in get_usertab_name.
Rewrote the code from the old usertab file to blend it cleanly with the ldap code.
Added btree_cache.h that was missing from the repository.
Added const declaration where appropriate in btree_cache.
Fixed the inclusion of the ldap headers to compile without the local declarations in defs.h.