pkg: parsers: add /etc/passwd parser

This patch adds a parser for /etc/passwd-formatted data (and unit tests
for said parser).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
daemon: *: export GetResourcePath and GetRootResourcePath

This allows for external packages to *safely* attain the full path of
any resource in a container's rootfs or base.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
builder: allow the USER directive to modify the owner of COPY'd files

This patch ensures that the USER directive will change the owner of any
COPY'd files or directories to the user id and primary group id of the
user which the builder's run config currently runs as. The user and
primary group ids are ascertained by inspecting the latest container's
/etc/passwd file (which is parsed on each COPY directive).

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