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

This patch adds an 'os/user'-like API to 'user/', allowing callers to
easily search for user and group entries in the operating system's
'/etc/passwd' and '/etc/group' files without needing to open file handles
and creating filter function closures. Some of this functionality is
lacking in the Go standard library (mainly group entry lookups).

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