Makes `dscl` usage service-agnostic

To accommodate systems managed by, e.g., Active Directory, check for
user in the search path.
Merge pull request #2124 from bak/fix_for_directory

Makes `dscl` usage service-agnostic when checking for user shell
remove -Z from rbconfig when installing binary rubies - prevent problems with Xcode libraries search path on target system, fix #2125
Merge branch 'master' of github.com:wayneeseguin/rvm