user: added a os/user-like API for quickly finding user entries

This patch adds an 'os/user'-like API to 'user/', allowing callers to
easily search for user entries in the operating system's '/etc/passwd'
file without needing to open file handles and creating filter function
closures.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)