gdb: split no_shared_libraries, command vs implementation

gdb: split no_shared_libraries, command vs implementation

The `no_shared_libraries` function is currently used to implement the
`nosharedlibrary` command, but it also used internally by other
functions.  This does not make a very good internal API.

Add the `no_shared_libraries_command` function to implement the CLI
command.  Remove the unused parameters from `no_shared_libraries`.

Remove the `from_tty` parameter of `target_pre_inferior`, since it's now
unused.

Change-Id: I4fcba5ee1e0f7d250aab1a7b62b9ea16265fe962
Approved-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Thiago Jung Bauermann <thiago.bauermann@linaro.org>