Bionic changes for tty related applets

Tested (but applets not included/enabled)
getty, login, su, 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 login allow also to do that :
Example: "busybox login shell"

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

Update also selinux related stubs after tests on x86 4.4.2,
l-preview and aosp/master

If you want to include these applets, here is the required
source list to add in busybox-full.sources :

 libbb/correct_password.c libbb/pw_encrypt.c libbb/update_passwd.c
 loginutils/getty.c loginutils/login.c loginutils/su.c
 loginutils/passwd.c loginutils/chpasswd.c loginutils/cryptpw.c

And the config flags to add :

 CONFIG_GETTY=y
 CONFIG_LOGIN=y
 CONFIG_FEATURE_NOLOGIN=y
 CONFIG_PASSWD=y
 CONFIG_CRYPTPW=y
 CONFIG_CHPASSWD=y
 CONFIG_SU=y
 CONFIG_FEATURE_SU_CHECKS_SHELLS=y

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

Change-Id: I96ee64f0872856fad6b3ff299faafad6451da556