gitignore: expand list of ignored files

Add the *.swp and *~ pattern for vim temporary files. Expand the
cscope ones to include all files possibly generated by cscope and also
add some known object formats.

Also remove the leading '?' from cscope.out, or else it doesn't match
the cscope.out file generated by default (as it expects an extra
character in front).

Reviewed by: gjb, uqs, lwhsu
Sponsored by: Citrix Systems R&D
Differential Revision: https://reviews.freebsd.org/D28095