cifs-utils: new plugin architecture for ID mapping code

cifs-utils: new plugin architecture for ID mapping code

Currently, the ACL-related tools in cifs-utils call into the wbclient
libs directly in order to do their bidding. The wbclient developers want
to get away from needing to configure winbind on the clients and instead
allow sssd to handle the mapping in most cases.

This patch represents an initial step in that direction. It adds a
plugin architecture for cifs-utils, adds wrappers around the calls into
libwbclient that find an idmap plugin library to use and then has it
call into that plugin to do the actual ID mapping.

The application will call into a set of routines that find the correct
plugin and dlopen() it. Currently the plugin is located in a well-known
location that is settable via autoconf. That location is intended to be
a symlink that points to the real plugin (generally under $pkglibdir).

The plugin will export a number of functions with well-known names. The
wrappers find those by using dlsym() and then call them.

Signed-off-by: Jeff Layton <jlayton@samba.org>