Only link against libiconv on darwin and freebsd; other OSes appear to provide iconv as part of libc.

Previously we read one, fixed buffer of input from the console; but this could
cause a problem if a multibyte character crossed the end of the buffer boundary.

Now, we detect that case from iconv and recover by reading one byte of input at
a time until the conversion succeeds.

Now that we don't use stm, we have to be careful about locking.  Specifically,
writeList2Chan is not atomic, so we need to add an explicit lock around it.

Without this patch, the bug can be reproduced by pasting a string of 50-60
characters into the input.

Don't provide the String -> String encoding functions; it's bad practice and they're easy to reproduce if needed.