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

usr.sbin/adduser/adduser.sh

@@ -733,10 +733,10 @@ input_interactive() {
 					trap 'stty echo; exit' 0 1 2 3 15
 					stty -echo
 					echo -n "Enter password: "
-					read -r upass
+					IFS= read -r upass
 					echo''
 					echo -n "Enter password again: "
-					read -r _passconfirm
+					IFS= read -r _passconfirm
 					echo ''
 					stty echo
 					# if user entered a blank password