Log: scheduler: show flag name in more trace messages

pe_action_set_flag_reason() could previously take either a pe_action_t*
argument or a const char *text argument.

Now, replace the text argument calls with direct calls to
pe_clear_action_flags() and pe_action_set_reason(), which makes the intent more
obvious and shows the flag name in the trace message.

This also allows us to simplify pe_action_set_flag_reason() further.