manage --cmd argument multi-times on commandline

Allow starting vimb with multiple --cmd arguments on commandline.

- convert vb.config.autocmd to GList
- process all the list at vimb startup
- pass all the list to new process on opening a new target
- cleanup on exit
multi-cmd: update documentation
multi-cmd: use GSList instead of GList

the argument building (at launch time) is not optimal, as arguments are
appended (instead of prepended). But as it is a one shot per instance,
and there should not have lot of --cmd passed, it should be tolerable.
rename autocmd var, used for --cmd processing

rename vb.config.autocmd to vb.config.cmdargsi, which is a little bit
missleading cause of the autocmd feature.