Deal with the inferior unloading shared objects.

Deal with the inferior unloading shared objects.
* solib.c (current_sos): New function, replacing find_solib.
(find_solib): Deleted.
(free_so): New function.
(clear_solib): Call free_so, instead of writing it out.
(solib_add): Rewritten: compare the inferior's current list of
shared objects with GDB's list, and do the required loads and
unloads.
(info_sharedlibrary_command, solib_address): Don't use find_solib
to walk the list of shared libraries: call solib_add, and then
walk the list at so_list_head normally.
* objfiles.c (free_objfile): Don't call CLEAR_SOLIB, and don't
detach the core target.  These tasks are taken care of elsewhere.
* target.c (remove_target_sections): New function.
* target.h (remove_target_sections): New declaration.

* solib.c (symbol_add_stub): Check whether we've already created
an objfile for this shared object first, before doing all that
work to compute section addresses, etc.

* objfiles.c (unlink_objfile): Report an internal error if objfile
doesn't occur in the object_files list.

* solib.c (special_symbol_handling): Delete argument; it's not
used.

* solib.c (SOLIB_EXTRACT_ADDRESS):  New macro to extract addresses
from solib structures. Use it throughout solib.c, get rid of all
CORE_ADDR casts.
(struct so_list):  Change type of lmaddr to CORE_ADDR.
(first_link_map_member):  Change return value type to CORE_ADDR,
update callers.
(solib_add_common_symbols):  Change parameter type to CORE_ADDR,
update callers.
(open_symbol_file_object, find_solib):  Change type of lm variable
to CORE_ADDR.