Allow only one arg for --arch/-a

Allow only one arg for --arch/-a

Allowing multiple args for -a/--arch causes some trouble (depending on the order)
E.g. "eshowkw cat/pkg -a a b" would work while "eshowkw -a a cat/pkg" wouldn't
because it would count cat/pkg to the -a/--arch option.

eshowkw -a amd64 kdelibs
No ebuilds at "/root"