build: Clarify output of 'make help'.
"attach method" is from now on known as "backend".

This large, but mainly mechanical commit, renames "attach method"
everywhere to "backend".

Backwards compatibility of the API (guestfs_{set,get}_attach_method)
and environment (LIBGUESTFS_ATTACH_METHOD) is maintained, but in new
code use guestfs_{set,get}_backend and LIBGUESTFS_BACKEND instead.

The default backend (launching qemu directly) is now called 'direct'
instead of 'appliance', although you can still use 'appliance' as a
synonym.
valgrind: Write separate log files in tmp/valgrind-<DATE>-<PID>.log

Current code wrote a single valgrind.log in the top-level directory.
This was overwritten with each valgrinded program, which wasn't much
use.  The new version writes multiple separate valgrind-*.log files
containing the date and PID (so each is unique).

It would nice if valgrind could delete log files that don't contain
errors, but this doesn't appear to be possible.
make clean: Clean contents of tmp directory.
make clean: Remove *~ (editor backup) files.
build: Change various check rules so they implicitly act like 'make -k'.

Rules such as 'make check-valgrind' are implemented using a for-loop.
They would always exit after the first error, even if 'make -k' was
used at the top-level.  Since 'make -k'-style behaviour is generally
more useful, change these for-loops so they run all the tests, and
report errors at the end.