Fix remaining bionic warnings

Tested on arm and x86 (kitkat),
aosp/master has some other ones

Change-Id: I170b16f2e091b99f971fb518bf0106c0e92adfdd
Test some tty related applets (getty, login, su)

And passwd, mkpasswd, cryptpw

Sample /etc/passwd required before 'passwd' :
root:x:0:0:root:/:/system/xbin/bash

busybox su allow to change current user, if you are root
Example : "busybox su shell -C /system/xbin/bash"

busybox su is not symlinked to prevent problems with
the optional su daemon

xgetpwnam() returns now an empty string instead of null,
busybox often check directly pw->pw_passwd[0]

PS: This commit was made to test applets on bionic,
and will maybe not be merged as-is later (new applets)

Change-Id: I96ee64f0872856fad6b3ff299faafad6451da556