libthr: thr_attr.c: More style and clarity fixes

The change of argument for sizeof() (from a type to an object) is to be
consistent with the change done for the malloc() code just above in the
preceding commit touching this file.

Consider bit flags as integers and test whether they are set with an
explicit comparison with 0.

Use an explicit flag value (PTHREAD_SCOPE_SYSTEM) in place of a variable
that has this value at point of substitution.

All other changes are straightforward.

Suggested by: kib
Reviewed by: kib
Approved by: emaste (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D43327