gdb: fix stale references to so_list

Adjust some comments and function names that refer to the ancient
so_list type.

Update some other stale comments in solib-svr4.c at the same time.

Change-Id: Ia42efa6554d0cc6abb4183b6bffc96c6358c5735
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>
11 files changed