watchmaildir: support multiple watchheader values
The watchheader key supports only a single value. Supporting multiple
watchheader values was mentioned in discussion [1] of
8d3e3bd8 (doc:
explain publicinbox.<name>.watchheader, 2019-10-09), and it wasn't
clear if there was a need.
One scenario in which matching multiple headers would be convenient is
when someone wants to set up public-inbox archives for some small
projects but does _not_ want to run mailing lists for them, instead
allowing others to follow the project by any of the pull mechanisms.
Using a common underlying address, an address alias for each project
is configured via a third-party email provider, with messages for each
alias being exposed as a separate public-inbox archive. In this
setup, messages for an inbox cannot be selected by a List-ID header
but can be identified by the inbox's address in either the To or Cc
header.
To support such a use case, update the watchheader handling to
consider multiple values, accepting a message if it matches any value.
While selecting a message based on matching _any_ rather than _all_
values is motivated by the above scenario, it's worth noting that the
"any" behavior is consistent with how multiple listid config values
are handled.
[1] https://public-inbox.org/meta/
20191010085118.r3amey4cayazfycb@dcvr/