base: add remove_client to rm_session

Fixes #13
codezero: fix compiler warnings int/unsigned

Warnings like the following:

warning: narrowing conversion of ‘((Genode::Platform_pd*)this)->Genode::Platform_pd::_space_id’ from ‘int’ to ‘Codezero::l4id_t {aka unsigned int}’ inside { } is ill-formed in C++11 [-Wnarrowing]