vim: Handle selections in move_to_line_n

Motion commands should modify a selection (if any) rather than clear it;
while stuff like 'd10G' is uncommon, supporting it does not hurt ...

https://bugzilla.gnome.org/show_bug.cgi?id=737973
vim: Fix selection-bound off-by-one error in move_line_start

Just like the fix to forward-word in commit 61d5432405, but for '^'
this time ...