Use `extern` where appropriate so that we can remove `-fcommon`
on macOS and others.  On macOS, `-fcommon` generates a number of
linker warnings.

I've tested building on macOS, Linux, FreeBSD, NetBSD, OpenBSD
and illumos.  I am not in a position to test on AIX or other
more esoteric platforms, however.

MacFUSE 4 removes support for passing device fd to the mount command. Adds
support for the receiving the fd over a socket instead, and updates command paths
and filesystem name.

Programs that want to background themselves now need
to define threadmaybackground returning 1.
This avoids a confusing (to people and debuggers)
extra parent process for all the threaded programs
that will never want to background themselves.

f is open, so walk will fail.
The comments say we only need one directory anyway.

Fixes #277.

Improved error message in case of unexpected open flags. The message

    unexpected open flags requested=0100040 unhandled=040

prompted me to clear the FMODE_EXEC flag, although I wonder if I
shouldn't have set OEXEC (0x3) instead.

Remote whitespace at the ends of lines.
Remove blank lines from the ends of files.
Change modes on source files so that they
are not executable.

Signed-off-by: Dan Cross <cross@gajendra.net>

MacPorts installs osxfuse under /opt/local.

read(3) sometimes errors with EINTR on macOS over slow connections.
9pfuse(1) now retries read(3) instead of sysfatal(3)ing.

Fixes #81.