MFC r318304: getusershell: don't write paste end of buffer reading shells

MFC r318304: getusershell: don't write paste end of buffer reading shells

_local_initshells did not reset cp to the beginning of the line buffer for
every iteration that it called fgets(3), leading to writing past the end of
line with fairly long /etc/shells or excessively long line lengths. Correct
this by properly resetting cp.

PR:		192528