Ensure that getpwent() is used in setpwent(), getpwent(),

Ensure that getpwent() is used in setpwent(), getpwent(),
endpwend() sequences (ditto for getgrent(), getspent(), and
getsgent()). The only real (minor) issue was in login, which kept
the passwd file open.
* libmisc/entry.c: Remove unneeded setspent() and endspent() (only
  getspnam is called in the middle).
* libmisc/find_new_ids.c: Make sure to close the password and
  group files with endpwent() and endgrent().
* libmisc/pwdcheck.c: Remove unneeded endspent() (only getspnam()
  is called before).
* src/lastlog.c, src/passwd.c, src/groupmod.c, src/faillog.c,
  src/groups.c: Make sure to close
  the password file with endpwent().
* src/login.c: Remove unneeded setpwent() (only xgetpwnam is
  called before).
* src/login.c, src/newgrp.c: Fix typos in comments.