src/logoutd.c: Use STRNDUPA() instead of its pattern

src/logoutd.c: Use STRNDUPA() instead of its pattern

STRNDUPA() is equivalent to automatic storage allocation (alloca(3))
+ ZUSTR2STP().

The benefits of this refactor are:

-  The allocation size is always correct, and needs no comments, since
   it's now automatically calculated by the macro.

-  STRNDUPA() is probably more familiar, since
   -  strndupa(3) is a libc function,
   -  STRNDUPA() is the obvious wrapper that
      calculates the size based on the input array.

-  We can remove ZUSTR2STP().

Signed-off-by: Alejandro Colomar <alx@kernel.org>