lib: lib-signals: Removed explicit notification of ioloop change through lib_signals_reset_ioloop() in favor of implicit method. Before, if the ioloop changed, the application had to explicitly notify lib-signals using lib_signals_reset_ioloop(). This is error-prone and requires doing this all over the Dovecot code base. Now, lib-signals registers an ioloop switch callback that deals with this implicitly. The application can detach lib-signals from the ioloop explicitly if delayed signal handling is not required/desired in the new ioloop. Specific delayed signal handlers can be exempt from this automated behavior using a flag, meaning that such signal handlers need to be moved between ioloops explicitly.

Removed CONTEXT_TYPE_SAFETY macro and reimplemented its functionality better. gcc/clang now gives a compiler error in many places if callback isn't exactly what was expected. It's also now much easier to add more of these checks.

Marked functions parameters that are allowed to be NULL. Some APIs were also changed. The non-obvious APIs where NULL parameter was changed to "" are master_service_init() and auth_master_user_list_init(). These checks can currently be enabled only on a patched clang: http://llvm.org/bugs/show_bug.cgi?id=6786

Added API for waiting child processes. --HG-- branch : HEAD