gdb/cli: add '.' as an argument for 'list' command

gdb/cli: add '.' as an argument for 'list' command

Currently, after the user has used the list command once, there is no
self-contained way to ask GDB to print the location where the inferior is
stopped.  The current best options require either using a separate
command to scope out where the inferior is stopped, or using "list *$pc"
requiring knowledge of GDB standard registers.  This commit adds a way
to do that using '.' as a new argument for the 'list' command.  If the
inferior isn't running, the command prints around the main function.

Because this necessitated having the inferior running and the test was
(seemingly unnecessarily) using printf in a non-essential way and it
would make the resulting log harder to read for no benefit, it was
replaced by a different statement.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>