This commit does not change a user visible behaviour. It refactors pwd
module to explicitly divide code that fetches data from passwd / group
databases from code that performs deserialization of the data to Lua
tables.

The idea of splitting of those actions appears when it was observed that
a call of getpw() / getgr() leads to problems on some systems when it is
invoked during passwd / group database traveral.

Now it is more obvious that we don't call getpw() during passwd
traversal and getgr() during group traveral.

Follows up #4428 and #4447.