doc: login-utils/*: Fix some warnings from "mandoc -T lint"

doc: login-utils/*: Fix some warnings from "mandoc -T lint"

mandoc: ./login-utils/chfn.1:96:2: WARNING: skipping paragraph macro: PP empty

mandoc: ./login-utils/lslogins.1:14:2: WARNING: skipping paragraph macro: PP after SH

mandoc: ./login-utils/newgrp.1:18:2: WARNING: skipping paragraph macro: PP empty

mandoc: ./login-utils/nologin.8:19:2: WARNING: skipping paragraph macro: PP empty

mandoc: ./login-utils/sulogin.8:57:2: WARNING: skipping paragraph macro: PP empty

mandoc: ./login-utils/utmpdump.1:61:2: WARNING: skipping paragraph macro: PP after SS

mandoc: ./login-utils/vipw.8:65:2: WARNING: line scope broken: TP breaks I

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  There is no change in output from "nroff" or "groff".

Signed-off-by: Bjarni Ingi Gislason <bjarniig@rhi.hi.is>