iconv: Support in-place conversions (bug 10460, bug 32033)

iconv: Support in-place conversions (bug 10460, bug 32033)

Check if any of the input files overlaps with the output file, and use
a temporary file in this case, so that the input is no clobbered
before it is read.  This fixes bug 10460.  It allows to use iconv
more easily as a functional replacement for GNU recode.

The updated output buffer management truncates the output file
if there is no input, fixing bug 32033.

Reviewed-by: DJ Delorie <dj@redhat.com>