Automatic announcement generation by calm

[Jon Turney <jon.turney@dronecode.org.uk>]

This is an experimental facility, currently only available for packages 
deployed from the build service [1] (that is, not for self-built 
packages uploaded with 'cygport up' via sftp)

When the token "announce" is present for a build job (in addition to 
"deploy"), after a successful deploy, calm will automatically generate 
and send an announce email.

The mail follows a similar format to that generated by "cygport 
announce", containing a list of packages and the description, with the 
following addition:

* If the cygport defines the variable "ANNOUNCE", it's evaluated 
contents will be appended to the generated mail.

* Otherwise, if the source package contains an ANNOUNCE file [2], it's 
contents will be appended.

* Otherwise, if the source package contains a README or ${PN}.README 
file, lines that look like part of a changelog, between one starting 
with '---- ${PVR}' and the next starting '----', will be extracted and 
appended

(None of these seem like a particularly great way of doing things, but 
they match some historical patterns.  As always, suggestions about 
improvements are welcome.)


In accordance with our long-standing policy of treating maintainer email 
addresses as private information, the mail is sent from cygwin-no-reply 
and bcc'ed to the uploader.


For testing purposes, if the token "mock" (yes, I am running out of 
synonyms for "test"...) is also present, the mail will be only sent to 
the uploader, not the announce list.


[1] https://cygwin.com/packaging/build.html
[2] Note that this isn't currently part of the default value of 
CYGWIN_FILES [3], so needs to be explicitly listed there to be included 
in the source package
[3] https://cygwin.github.io/cygport/src_prep_cygpart.html#CYGWIN_FILES