user: *: refactor and expand libcontainer/user API

This patch refactors most of GetUserGroupSupplementaryHome and its
signature, to make using it much simpler. The private parsing ftunctions
have also been exposed (parsePasswdFile, parseGroupFile) to allow custom
data source to be used (increasing the versatility of the user/ tools).

In addition, file path wrappers around the formerly private API
functions have been added to make usage of the API for callers easier if
the files that are being parsed are on the filesystem (while the
io.Reader APIs are exposed for non-traditional usecases).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
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)
Merge pull request #158 from cyphar/refactor-expose-user

Refactor and expose private functions within `libcontainer/user`.