s4-ldb: support a new type of ldb module loading

s4-ldb: support a new type of ldb module loading

this supports module loading in ldb which uses the approach of "load
all modules in a directory". This is much more flexible than the
current module loading, as it will allow us to load modules for
command line parsing and authentication.

Modules are loaded from a colon separated path, in the environment
variable LDB_MODULES_PATH. If unset, it defaults to LDB_MODULESDIR.

Within each directory modules are loaded recursively (traversing down
the directory tree). The device/inode number of each module is
remembered to prevent us loading a module twice.

Each module is checked for a ldb_init_module() function with
dlsym(). If found, it is called with the ldb module version as an
argument.