Add loadable module support #27
Description
Avoid hard dependencies on some shared libraries, such as libspf2 and libjssql. The corresponding Mailfilter modules shall be loaded by Sys.loadModule(), which is currently implemented as a stub in js_sys.c.
For libjssql, RDBMS-specific implementations, such as libjssql_mysql.so and libjssql_pgsql.so must also be loaded dynamically to avoid hard dependencies on the RDBMS client libraries.