disable colours by default in clang

make '-fno-color-diagnostics' the default in clang. to enable colours,
use -fcolor-diagnostics.

ok natano@ patrick@ jcv@
add patch for 35e66e842e; requested by patrick@
Remove *.diff from .gitignore.

As we have .diff files in our tree (contrib/llvm/patches),
remove *.diff from .gitignore, so that git acknowledges
their existence.

ok natano@ pedro@
retire mkstr(1) and xstr(1); no objections
Enable tmpfs on armv7 ramdisk.
If the only process in the pgrp is in the middle of exiting, it might
not have any threads left.  Treat that the same as an empty pgrp.

encountered by and ok deraadt@