session: convert to C

C++ features are not used in this file.

Note, we need to move ifdef(__cplusplus) in user.h to make guest_user
and admin_user variables accessible from C code.

Also, we need to move initialization of session_vtab_registry to
session_init(), because most C compilers don't allow to initialize
a global variable with a value of another global variable.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring