argument parsing refactor, add shared options

[David Bremner]

As part of ongoing hacking on Austin's revision tracking patches, I
wanted to add an option to (almost) all subcommands. One thing led to
another and this series emerged. By itself, the gain in functionality
is probably not worthwhile (yay, we can type notmuch subcommand
--version), but internally it makes it easy to add further "global
options" that are accepted as

% notmuch --option [subcommand]

and

% notmuch subcommand --option

it would make sense, e.g. for --quiet and --verbose to be supported
this way.

Whatever people think about the shared options, I think the
refactoring of notmuch_help_command is probably worthwhile as it is
pretty simple and improves the readability of that code.

Although there are other ways of doing so, patch 4 of this series also
fixes a UI bug encountered by Rob recently, where "--config" is silently ignored
in

	notmuch setup --config=/tmp/foo.conf

In principle this series will need a documentation update, but note that

   notmuch search --help

is already undocumented.