macro: add new assert_return() macro for early parameter checking in functions

For the library functions we expose we currently repeatedly use checks
like the following:

if (!value_is_ok(parameter1))
        return -EINVAL;
if (!value_is_ok(parameter2))
        return -EINVAL;

And so on. Let's turn this into a macro:

assert_return(value_is_ok(parameter1), -EINVAL);
assert_return(value_is_ok(paramater2), -EINVAL);

This makes our code a bit shorter and simpler, and also allows us to add
a _unlikely_() around the check.
event: implement quit handlers

Quit handlers are executed when an event loop is terminated via
sd_event_request_quit(). They are in a way atexit() handlers that are
executed in a well-defined environment, time and thread: from the event
loop thread when the event loop finishes.
event: rename the "mute" field of event sources to "enabled"

In addition, the states "UNMUTED" and "MUTED" become "ON" and "OFF".

This has the benefit that a possible value of this field is not
identical to its name, thus minimizing confusion.
event: add sd_event_source_get_child_pid() call to query the PID of a child event source
event: move all library calls over to new assert_return() macro