win32.c: user name

* win32/win32.c (init_env): save USER environment variable in
  login name if it is set.  [ruby-core:66163] [Bug #10493]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48364 b2dd03c8-39d4-4d8f-98ff-823fe69b080e