[build] minor compilation issues

[build] minor compilation issues

SuSE checker found some minor issues:

- refwwid in libmultipath/configure.c:get_refwwid() might be used
uninitialized. As dev_type is a simple 'int' there is nothing which
prevents the unsuspecting user to call it with an unhandled value.
We shoud rather use an enum and set refwwid to NULL to be on the safe
side.

- attr in libmultipath/discovery.c:devt2devname() might be used
uninitialized. Quite unlikely, but might happen in principle.
So better have it initialized.

- r in multipath/main.c:main() might be used uninitialized.
True. So have it initialized.