Use realpath to canonicalize the config path before writing.
Previously 'notmuch setup' and 'notmuch config set' overwrote the
config file even if it was a symbolic link.
There are currently 45 TESTS scripts. 36 of those load
test-lib.sh using '. ./test-lib.sh' and 9 '. test-lib.sh'.
In latter case test-lib.sh is first searched from directories
in PATH (posix) and then from current directory (bash feature).
Changed the 9 files to execute '. ./test-lib.sh'. The test-lib.sh
should never be loaded from directory in PATH.
Add a command to list all configuration items with their associated
values.
One use is as follows: a MUA may prefer to store data in a central
notmuch configuration file so that the data is accessible across
different machines, e.g. an addressbook. The list command helps
to implement features such as tab completion on the keys.
