defer system_update - opensuse + unified handling
defer system_update - ubuntu
defer system_update - arch
defer system_update - centos, refactored requirements_check_fallback
defer system_update - freebsd
defer system_update - gentoo*
defer system_update - openbsd, extract __rvm_requirements_run_custom
defer system_update - openindiana
defer system_update - smartos
defer system_update - solaris
defer system_update - osx + smf, remove old update-system code
defer system_update - osx_brew
defer system_update - osx_fink
defer system_update - osx_port
Merge pull request #1833 from wayneeseguin/features/deffered_update

defer updating system after missing dependencies found
ask where before installing homebrew, closes #1839