This patch works towards to goal of common code shared between idmap_ldap

This patch works towards to goal of common code shared between idmap_ldap
and pdb_ldap.

So far, it's just a function rename, so that the next patch can be a very
simple matter of copying functions, without worrying about what changed
in the process.

Also removes the 'static' pointers for the rebind procedures, replacing them
with a linked list of value/key lookups.  (Only needed on older LDAP client
libs)

Andrew Bartlett
(This used to be commit f93167a7e1c56157481a934d2225fe19786a3bff)