Get rid of the global "base_target" and use "find_target_beneath"

    Get rid of the global "base_target" and use "find_target_beneath"
        to find the underlying target.
        * dec-thread.c (base_target): Delete.
        (enable_dec_thread): Remove assignement to base_target.
        (dec_thread_detach, dec_thread_wait, dec_thread_wait)
        (dec_thread_fetch_registers, dec_thread_store_registers)
        (dec_thread_mourn_inferior, dec_thread_pid_to_str):
        Update the function profile if necessary.
        Use find_target_beneath to call the same method but from
        the underlying target, removing the need for "base_target".

        * dec-thread.c (dec_thread_get_regsets, dec_thread_set_regsets):
        Fix a copy/paste error in a few debug traces...

        * solib-osf.c (init_so): Use a simpler method for computing
        the size of lm_info structure. This also gets rid of warning
        emitted by the compiler.