deluser: check if specified home is a directory before removing it

deluser: check if specified home is a directory before removing it

On Alpine, some users use /dev/null as a home directory. When removing
such a user with `deluser --remove-home` this causes the /dev/null
device file to be removed which is undesirable. To prevent this pitfall,
check if the home directory specified for the user is an actual
directory (or a symlink to a directory).

Implementations of similar tools for other operating systems also
implement such checks. For instance, the OpenBSD rmuser(1)
implementation [0].

[0]: https://github.com/openbsd/src/blob/b69faa6c70c5bfcfdddc6138cd8e0ee18cc15b03/usr.sbin/adduser/rmuser.perl#L143-L151

function                                             old     new   delta
deluser_main                                         337     380     +43

Signed-off-by: Sören Tempel <soeren+git@soeren-tempel.net>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>