nix-shell: Barf if -p and -E are both used
nix-shell: Barf if -p and -E are both used

Closes #454, #455.

dtzWill/nix,rycee/nix,offlinehacker/nix,bjornfor/nix,dtzWill/nix,bjornfor/nix,ttuegel/nix,gebner/nix,acowley/nix,gridaphobe/nix,rycee/nix,aucampia/nix,garbas/nix,ehmry/nix,rowhit/nix,rowhit/nix,dtzWill/nix,joelmo/nix,ehmry/nix,gridaphobe/nix,acowley/nix,wmertens/nix,offlinehacker/nix,rycee/nix,zimbatm/nix,bennofs/nix,peti/nix,acowley/nix,garbas/nix,vizanto/nix,peti/nix,offlinehacker/nix,vcunat/nix,tchajed/nix,peti/nix,ehmry/nix,dtzWill/nix,NixOS/nix,ttuegel/nix,bennofs/nix,ttuegel/nix,layus/nix,garbas/nix,gebner/nix,vizanto/nix,zimbatm/nix,wmertens/nix,layus/nix,gridaphobe/nix,rowhit/nix,vcunat/nix,aucampia/nix,wmertens/nix,shlevy/nix,ysangkok/nix,vcunat/nix,ttuegel/nix,wmertens/nix,pikajude/nix,acowley/nix,shlevy/nix,zimbatm/nix,vcunat/nix,joelmo/nix,layus/nix,dezgeg/nix,offlinehacker/nix,rowhit/nix,tchajed/nix,ehmry/nix,peti/nix,bjornfor/nix,aucampia/nix,ysangkok/nix,shlevy/nix,dtzWill/nix,garbas/nix,bennofs/nix,vizanto/nix,rycee/nix,dezgeg/nix,ehmry/nix,shlevy/nix,layus/nix,gebner/nix,NixOS/nix,aucampia/nix,bennofs/nix,NixOS/nix,ehmry/nix,peti/nix,pikajude/nix,dezgeg/nix,vizanto/nix,pikajude/nix,gebner/nix,zimbatm/nix,shlevy/nix,joelmo/nix,NixOS/nix,NixOS/nix,ysangkok/nix,tchajed/nix,joelmo/nix,dezgeg/nix,layus/nix,bjornfor/nix,gridaphobe/nix,dezgeg/nix,rycee/nix,garbas/nix,tchajed/nix