No more mhshow-charset-* but instead automatic invocation of iconv(1).

No more mhshow-charset-* but instead automatic invocation of iconv(1).
Unless there is an mhshow-show-* rule, text/plain content types are
automatically piped through iconv(1) if their charset is foreign.
For mhshow-show-* rules, there's a new %c escape, which expands to the
foreign charset. The contents of the files, referenced by %f/%F are still
in foreign encoding. One might want to use `iconv -f %c %f | mycommand'
for custom mhshow-show-* rules.
We depend on iconv(1) now! But this may change to dependence on iconv(3),
soon.
This field is inconsistent and not at all finished, yet.