gdb: rename struct shobj -> struct solib

gdb: rename struct shobj -> struct solib

`struct so_list` was recently renamed to `struct shobj` (in 3fe0dfd1604f
("gdb: rename struct so_list to shobj")).  In hindsight, `solib` would
have been a better name.  We have solib.c, the implementations in
solib-*.c, many functions with solib in their name, the solib_loaded /
solib_unloaded observables, etc.

Rename shobj to solib.

Change-Id: I0af1c7a9b29bdda027e9af633f6d37e1cfcacd5d
Approved-By: Tom Tromey <tom@tromey.com>