Avoid double slash when explicitly passing --root or --rootdir

Passing a path with a trailing slash to --root or --rootdir can lead to a
double slash at the start of paths.  e.g.

$ pacman --root / -v 2>1  | grep " //"
Log File  : //var/log/pacman.log

In MSYS2, paths starting with // will hit the network and fail.

Avoid this be explicitly stripping the trailing / from paths passed to these
flags.

Signed-off-by: Allan McRae <allan@archlinux.org>