Enable CI builds for MaxcOS on CirrusCI.

This adds support for MacOS (Monteray onward, although should work on older MacOSes). The BSD wrapper for poll/ppoll has been lifted from the work the folks over at OpenSSH have done -- thanks!

Move the detection of UUID header files to be included portably. This will have no functional effect until MacOS detection is put in place.

Update got-{fetch,index,send}'s Makefile.am to include ratelimit.c for progress output.

endian.h is portably included on non-OpenBSD systems, so let that mechanism determine if it should be included or not.

ok stsp

ok stsp

imsg_clear frees and closes resources allocated as part of enqueueing imsgs so it's a no-op after reads. discussed with and ok stsp@

As done elsewhere, d->namlen is not a portable field from readdir, therefore simualte this via the strlen of the directory name instead.

tested by thomas, naddy, and myself

Replace FMT_SCALED_STRSIZE with (FMT_SCALED_STRSIZE - 2) as field width when formatting output for printing. FMT_SCALED_STRSIZE includes space for a nul byte and a minus sign. Output values are expected to be always positive here. ok stsp

ok millert stsp

Add missing include for the deltify test now that it depends on murmurhash2.