imapd.c: properly handle SEARCH RETURN () as SEARCH RETURN (ALL)
ndex.c: Fix for bug #3853 - https://bugzilla.cyrusimap.org/show_bug.cgi?id=3853
Bug#3846: fix type in "nonexistent"

Replace "nonexistant" with "nonexistent".

The rest of the bug report is proceeded by commit 649faa3f852c993e37
on cyrus-imapd-2.4 and commit 49fd9a8c95a6340 on master.
lib/charset.c: mark internal functions as static

and remove some terminating whitespaces.

Backported from master
Bug#3848: support charset aliases in encoded headers

Treat
  Subject: =?UTF8?b?0JrQstC+IA==?=
and
  Subject: =?UTF-8?b?0JrQstC+IA==?=
in the same way.  Backport from master branch mkchartable.pl,
charset/aliases.txt and the support for charset aliases *in headers*.