vipw: print only one new line when asking about shadow file

vipw: print only one new line when asking about shadow file

Commit 11b86e1733 changed printf() to puts() in favour of more simple
function, but forgot that puts() adds a new line to end of string.  That new
line is neither needed, or expected, so use fputs() that is both a simple
printing function and comes without new line side effect.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>