su should reset PATH, IFS, LOGNAME, and USER.

Also add -h and --help while we're here (since all the toybox commands carry
their own documentation, and our su isn't much like the real su).

Bug: 19647373
Bug: 19564110
Change-Id: If2e6117b6701dcac03b5831f251e60a272bf0d61
1 file changed