Change maildir and mh check_mailbox to use dynamic sized hash. (closes #3973)
The original patch is by Matt Fleming, originally posted at
http://www.codeblueprint.co.uk/2017/01/15/a-kernel-devs-approach-to-improving2
The comments there indicate Matt tried to submit to trac and mutt-dev,
but ran into registration problems. Thank you for the patch, and
sorry for those problems, Matt.
I modified the patch by making the same change to the
mh_check_mailbox() code too.