MFC r321849,r321852:

r321849:

Fix the return type for pager_output(..) in bootparttest and zfsboottest

pager_output(..) has always returned int. For some reason (probably the
clang 5.0 update), this mismatch now results in an error with CC=clang.

r321852:

Fix up r321849

pager_output should return 0 if no more output needs to be paged; non-zero
otherwise.

MFC with: r321849