* observer.sh: Add struct so_list declaration.
* Makefile.in: Add dependencies on observer.h for solib.c and
breakpoint.c.
* breakpoint.c (disable_breakpoints_in_unloaded_shlib): New
function.
(_initialize_breakpoint): Register
disable_breakpoints_in_unloaded_shlib as an observer of the
"solib unloaded" observation event.
(re_enable_breakpoints_in_shlibs): For bp_shlib_disabled breakpoints,
call decode_line_1 so unfound breakpoint errors are silent.
* solib.c (update_solib_list): When a solib is discovered to have
been unloaded by the program, notify all observers of the
"solib unloaded" observation event.