Moved code that changes the pw_passwd entry (i.e shadow password and

Moved code that changes the pw_passwd entry (i.e shadow password and
weird unixware stuff) into _Get_Pwnam() to fix a memory allocation bug.

Note that the Get_Pwnam() function now returns a const struct passwd *
as a hint to other developers not to change entries in the struct
passwd.
(This used to be commit 36d7cb4ccc42268e8e6a7b783c945d1853624958)