doctests: Extract and use

Include the doctests from the toplevel Subdir.sd.mk.

In check, extract the doctests, and seddery the expected output into
the combined expected output file.  This involves adding a comment in
Subdir.mk.expected to indicate where the doctests will appear.

In update-expected, contrive to strip the doctest output back out of
the actual output so we can update only the manually-edited part.

Consequential changes to MAKEFILE_TEMPLATES in main.mk.expected
and .gitignore.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>