Add ability to read /etc/passwd and /etc/group

Add the capability to read /etc/passwd and /etc/group for getpw* and
getgr* functions.

Bug: 27999086
Test: pwd, grp, grp_pwd_file unit tests
Test: Read in custom users/groups from /etc/{passwd,group}
Change-Id: Idc1f054af8a7ca34743a90493495f0ccc775a0d8