client: Drop use of getpass()

client: Drop use of getpass()

getpass() is traditional Unix, but has been withdrawn from POSIX.  We
still use it when it's available, and fall back to portable code only
when it's not.

The portable code behaves differently: it reads stdin instead of
/dev/tty, and in noncanonical mode.

Simplify things: always use the replacement.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>