Add $crypt_protected_headers_save

Add $crypt_protected_headers_save

Setting this option will save the protected header back into the
clear-text message headers.  This improves
usability (searching/limiting/replying) when reopening a mailbox
without header cache.  However, it is a security trade-off, so
defaults off and strongly warns about what it is doing in the documentation.

Co-authored-by: Richard Russon <rich@flatcap.org>