Unlike the (current) infix query parser provided by Xapian, the
notmuch specific sexp query parser supports prefixed wildcard queries,
so use those. In addition to being somewhat faster, this avoids
needing to escape all of the user's tags to pass via the shell.
Although the code required to support both new and old environment
variables is small, it complicates the semantics of configuration, and
make the documentation harder to follow.
