lua: pwd: fix passwd and group traversal

CentOS 6 and FreeBSD 12 implementations of getpwuid() rewind
setpwent()-{getpwent()}-endpwent() loop to a start that leads to a hang
during pwd.getpwall() invoke. The same is true for a getgrgid() call
during setgrent()-{getgrent()}-endgrent() loop.

The commit modifies pwd module to avoid getpwuid() calls during passwd
database traversal and to avoid getgrgid() calls when traversing groups.

The commit also fixes the important regression on CentOS 6 after
f5d8331e ('lua: workaround
pwd.getpwall() issue on Fedora 29'): tarantool hungs during startup due
to added getpwall() call. This made tarantool unusable on CentOS 6 at
all.

Aside of that the commit fixes another pwd.getgrall() problem: the
function gaves password entries instead of group entries.

Fixes #4428.
Fixes #4447.
Part of #4271.