bionic: Implement getpwent and getgrent

Not efficient to iterate through given the large number of Android
ids (AID). Compile warning will result if you use these functions,
telling you as much. Not for general consumption, however for
example, some filesystem tests would like to see these to perform
all corners.

About 1/4 second for getpwent, and 1/8 second for getgrent to iterate
through all reserved Android aids.

Bug: 27999086
Change-Id: I7784273b7875c38e4954ae21d314f35e4bf8c2fc