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

This patch adds an 'os/user'-like API to 'user/', allowing callers to
easily search for group entries in the operating system's /etc/group'
file without needing to open file handles and creating filter function
closures. This implements a missing feature in Go's standard library in
pure Go.

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