Fixed for glibc iconv

One more issue for both libraries is how they handle 'einval' error when encoding specified with //IGNORE flag. See https://github.com/Vagabond/erlang-iconv/blob/master/c_src/iconv_drv.c#L190-L194
This is the reason of why 2 tests fall in gen_smtp: https://github.com/Vagabond/gen_smtp/blob/master/src/mimemail.erl#L1361 and https://github.com/Vagabond/gen_smtp/blob/master/src/mimemail.erl#L1512 with iconverl as well as with eiconv.

$ echo "this contains a copyright © symbol" | iconv  -f us-ascii -t utf-8
this contains a copyright iconv: illegal input sequence at position 26

$ echo "this contains a copyright © symbol" | iconv  -f us-ascii -t utf-8//ignore
this contains a copyright  symbol
iconv: illegal input sequence at position 36

$ echo "this contains a copyright © symbol" | iconv -f us-ascii -t utf-8//ignore 2>/dev/null
this contains a copyright  symbol

$ echo "this contains a copyright © symbol" | iconv -c -f us-ascii -t utf-8  # note '-c' switch
this contains a copyright  symbol

$ erl
1> eiconv:convert("us-ascii", "utf-8", <<"this contains a copyright © symbol">>).
{error,eilseq}
2> eiconv:convert("us-ascii", "utf-8//IGNORE", <<"this contains a copyright © symbol">>).
{error,eilseq}
3> iconverl:conv("utf-8//IGNORE", "us-ascii", <<"this contains a copyright © symbol">>).
{error,eilseq}