Increase buffer size, use bounds checking functions

Increase the size of the user/group buffer from 16 bytes
to 32 bytes. Some OEMs are creating usernames longer than
15 bytes, causing problems.

Use bounds checking functions when handling user/group
data, to avoid overflowing buffers.

Change-Id: I4a5824b819b0c37662ba4f33573af0d0e071b444