adduser: allow standard IFS characters in passwords

Notably, the default IFS contains space/tab, thus any leading/trailing
whitespace characters tend to be removed.

Set IFS= for just the read lines to mitigate this, allowing the user to be
less surprised when their leading/trailing spaces weren't actually captured
in the password as they are with other means of setting a user's password.

PR: 245342
Submitted by: dereks_lifeofadishwasher.com
Reviewed by: jilles
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24292