package/makedevs: use the rooted /etc/passwd and /etc/group

Currently, makedevs will query the host's /etc/passwd and /etc/group to
resolve usernames and group names. This is inherently flawed, as we can
never guarantee that the UIDs will be the same on the target as on the
host, or even whether a particular user does exist on the host.

This is because getpwnam() and getgrnam() will forcibly read the
system's /etc/passwd and /etc/group, and there is no way to tell them to
look anywhere else.

However, we can use fgetpwent() and fgetgrent() instead, for which
we can pass a FILE* stream to read from to get the entries. This means
we must implement the scanning-loop ourselves, but fortunately, that's
pretty trivial to do.

[Peter: swap errno / return value check, use bb_perror_msg_and_die, code style]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>