clang-format: Minor tweaks

Invert KeepEmptyLinesAtTheStartOfBlocks. We used to require an empty
line at the beginning of functions with no local variables, which I
believe is the reason for this setting. Now it is discouraged in new
code.

Tell clang-format to align consecutive macros, since we tend to do that.
clang-format's output isn't quite what we want here. Typically we have
a tab after a #define for some reason, and clang-format doesn't appear
to have an option for that. clang-format will also use a mix of tabs
and spaces to minimize indentation, which is also against our
convention. However, the result looks better with this setting than
without.

Reviewed by: emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29870