Extract LDAP values using get_ldap_values()

Move the extraction of LDAP values to separate function.
Rename init_pubkeys_from_ldap_values()

Rename init_pubkeys_from_ldap_values() to
extract_pubkeys_from_ldap_values().
White space clean up.
New user_attr configuration setting

The user attribute used in the LDAP query filter is now configurable.
The user attribute can be set via the `user_attr` setting in
`/etc/pkfs.conf`.

`user_attr` should match the LDAP attribute used to identify users with
SSH access. `user_attr` defaults to "uid".
Add comments for global configuration

It's now easy to tell where the global configuration is defined.