Encoded headers

This is the same as bug ID 3137120 (postponed and now closed), just updated for v.3.10.
I've added iconv check in configure.ac and re-encoding is now conditional - depends on iconv availability.

For autoconf stuff m4/iconv.m4 should be copied from glibc or gettext.

Just in case description from closed bug:

There are several problems (though minor) with emails with encoded headers:
- dspam only decodes headers in Base64 or QuotedPrintable, but ignores encoding, thus writes
data into logs in different charsets

- webui completely ignores encoded headers in Quarantine view and outputs whatever was written
into logs in History view.

Thus, for countries with many charsets (eg. Russia) its very difficult to view either Quarantine or History views - most Subjects and "From" lines are unreadable.

Attached patches convert everything into utf-8, thus solving this problems.