user: *: fix function signatures

This patch changes the incorrectly named functions GetPasswdFile,
GetGroupFile, GetExecUserFile and several internal components to
GetPasswdPath, GetGroupPath, etc -- as these are far more appropriate
names and more clearly convey to users what the arguments represent.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
*: merge to new user/ API

This patch switches libcontainer's usage of the user/ API to the new
version.

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