user: lookup: added os/user-like lookup API

This patch adds an os/user-like user lookup API, implemented in pure Go.
It also has some features not present in the standard library
implementation (such as group lookups).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
user: add unit tests for GetExecUser

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