fixup 9e7dc5ba008298d823c13cff214f7477601b5efd

The scoped_ptr<unit> construct which was used previously in
actions/create.cpp is unnecessary after this commit, and it seems
like it could be related to this error report:

http://wesnoth.org:8080/job/Wesnoth/branch=master,compiler=default,label=Debian7-64/185/consoleText

The most significant change of this commit is that
unit_creator::add_unit now does not make a "temporary" unit
instance from the cfg, and then copy construct it. From source
inspection that step appears to be unnecessary, and it somewhat
obfuscates the meaning of this code in actions/create.cpp
Merge branch 'master' of git://github.com/wesnoth/wesnoth