meta/pkg-build: add an "about" page

Explain `raco test`, the `PLT_PKG_BUILD_SERVICE` environment variable,
and the "x86_64-linux-natipkg" platform.
meta/pkg-build: record start and end time for tests
fd write: accomodate non-partial writes to a non-full descriptor

If a write to a non-blocking descriptor fails, then try again
with fewer bytes, since nothing in the spec write() seems to
promise writing partial amounts. In particular, writing to
a FIFO no Mac OS X might fail even if there are a few bytes of
space; as it happens, the select() function seems to compensate
and claim that such a FIFO is full, but kqeueue() doesn't.
fd read and write: avoid redundant O_NONBLOCK flag setting
work around a kqueue bug(?) on Mac OS X

There seems to be a problem in kqueue() on Mac OS X for watching for
FIFO write availability, where adding a read event for the same FIFO
(at a different file descritor) can disable the write event.

Merge to v6.1.1