Create an option to use LDAP

[yasn77]

This commit introduces a feature that gives the option to use LDAP. The implementation is through libpam-ldap and libnss-ldap. User home directories are automatically created using libpam-script, but SFTP_USERS variable will be ignored.

Further improvement would be to update files/create-sftp-user script, allow libpam-script to create user directories.