Auto merge of #1963 - Frederick888:master, r=alexcrichton
Auto merge of #1963 - Frederick888:master, r=alexcrichton

The most common case is that if `cargo` is not installed to  `/usr` but somewhere else such as `/usr/local` (is this the default install prefix? don't remember...), user will see `cargo: command not found` after `sudo su` executed.

P.S. sorry for previous wrong PR... there was a redundant commit.

Frederick888/cargo,tshepang/cargo,Kimundi/cargo,mark-buer/cargo,rustyhorde/cargo,AndiDog/cargo,AndiDog/cargo,gchp/cargo,sbeckeriv/cargo,aturon/cargo,vi/cargo,tsurai/cargo,tsurai/cargo,cesarb/cargo,blaenk/cargo,vojtechkral/cargo,winger/cargo,huonw/cargo,Ticki/cargo,mark-buer/cargo,iliekturtles/cargo,nipunn1313/cargo,brson/cargo,mark-buer/cargo,huonw/cargo,Frederick888/cargo,akiss77/cargo,achanda/cargo,achanda/cargo,tsurai/cargo,tshepang/cargo,eminence/cargo,ThinTim/cargo,aturon/cargo,rust-lang/cargo,Frederick888/cargo,blaenk/cargo,achanda/cargo,sbeckeriv/cargo,aturon/cargo,winger/cargo,beni55/cargo,Kimundi/cargo,shaleh/cargo,nipunn1313/cargo,iliekturtles/cargo,shaleh/cargo,eminence/cargo,Kimundi/cargo,brson/cargo,nipunn1313/cargo,winger/cargo,Ticki/cargo,vojtechkral/cargo,brson/cargo,brson/cargo,ArtemGr/cargo,Ticki/cargo,eminence/cargo,gchp/cargo,cesarb/cargo,fhahn/cargo,akiss77/cargo,beni55/cargo,achanda/cargo,fhahn/cargo,tshepang/cargo,vojtechkral/cargo,vojtechkral/cargo,aturon/cargo,cesarb/cargo,rust-lang/cargo,Ticki/cargo,AndiDog/cargo,Kimundi/cargo,huonw/cargo,ThinTim/cargo,gchp/cargo,rustyhorde/cargo,eminence/cargo,mark-buer/cargo,vi/cargo,ThinTim/cargo,Frederick888/cargo,winger/cargo,Kimundi/cargo,mark-buer/cargo,nipunn1313/cargo,ArtemGr/cargo,durka/cargo,durka/cargo,gchp/cargo,fhahn/cargo,sbeckeriv/cargo,ThinTim/cargo,rust-lang/cargo,AndiDog/cargo,vi/cargo,blaenk/cargo,iliekturtles/cargo,ArtemGr/cargo,sbeckeriv/cargo,beni55/cargo,shaleh/cargo,akiss77/cargo,iliekturtles/cargo,JacobHenner/cargo,winger/cargo,beni55/cargo,Ticki/cargo,cesarb/cargo,AndiDog/cargo,shaleh/cargo,cmr/cargo,huonw/cargo,aturon/cargo,vi/cargo,durka/cargo,tsurai/cargo,rustyhorde/cargo,akiss77/cargo,tshepang/cargo,cmr/cargo,huonw/cargo,ThinTim/cargo,Frederick888/cargo,cmr/cargo,rustyhorde/cargo,fhahn/cargo,akiss77/cargo,vi/cargo,JacobHenner/cargo,blaenk/cargo,rust-lang/cargo,ArtemGr/cargo,cmr/cargo,iliekturtles/cargo,cmr/cargo,beni55/cargo,JacobHenner/cargo,durka/cargo,JacobHenner/cargo,JacobHenner/cargo,brson/cargo,fhahn/cargo,ArtemGr/cargo,cesarb/cargo,shaleh/cargo,gchp/cargo,tsurai/cargo,blaenk/cargo