notmuch show: Fix to work with any search string rather than just a thread ID.

The more general command is more consistent, and more useful.

We also fix "notmuch search" to output copy-and-pasteable search terms
for the thread with "thread:" prepended already. Similarly, the
message-ID in the output of "notmuch show" is also now printed as a
valid search term, ("id:<message-id>" rather than "ID: <message-id>").

Naturally, the emacs code is also changed to track these changes.