vipw: make vim writebackup mode work

vipw: make vim writebackup mode work

Some editors, such as Vim with 'writebackup' mode enabled, use "atomic
save" in which the old file is deleted and a new one with the same name
created in its place.  The vipw tries to detect if such happen by
looking hard temporary file link count, when it is zero reopen
temporary file by using it's path.

Reported-by: Mantas Mikulėnas <grawity@gmail.com>
References: http://www.spinics.net/lists/util-linux-ng/msg06666.html
Signed-off-by: Sami Kerola <kerolasa@iki.fi>