tty-ask-password: copy argv[] before forking child

tty-ask-password: copy argv[] before forking child

Another fix in style of bd169c2be0fbdaf6eb2ea7951e650d5e5983fbf6.

Let's also avoid strjoina() in a loop (i.e. stack allocation). While in
this specific caseone could get away with it (since we'd immediately
afterwards leave the loop) it's still ugly, and every static checker
would be totally within its rights to complain.

Also, let's simplify things by not relying on argc, since it's redundant
anyway, and it's nicer to just treat things as NULL terminated strv
array.

Fixes: #12180