* doc/observer.texi (executable_changed): New observer.

    * doc/observer.texi (executable_changed): New observer.
        * symtab.c: Include "observer.h".
        (find_main_name): New function.
        (main_name): If name_of_main is unset, then compute it
        using find_main_name.
        (symtab_observer_executable_changed): New function.
        (_initialize_symtab): Attach executable_changed observer.
        * exec.c: Include "observer.h".
        (exec_file_attach): Emit executable_changed notification.
        * symfile.c: Include "observer.h".
        (reread_symbols): Send an executable_changed if appropriate.
        * Makefile.in (exec.o): Add dependency on observer.h.
        (symfile.o): Likewise.
        (symtab.o): Likewise.