Implement the passwd database subroutines with single-user database.
This database is populated with a <passwd/> sub-node of the libc
configuration node. All fields of the "passwd" struct may be specified
with reasonable defaults provided for a "root" user. This allows a
libc-based component to spoof user information for the sake of porting
existing Unix software.
A test is provided at run/libc_getpwent.
Fix#2919
