Various doc and message fixes.

Various doc and message fixes.
(non-iso-charset-alist): Add mac-roman.  Don't add entries for
codepages already present.
(list-block-of-chars): Display space for null entries in
translation table.  Display tab specially.
(list-non-iso-charset-chars): Check for null charsets.  Use pop
for clarity.  Don't break 8-bit sets into sections between ranges.
(list-charset-chars): Avoid indent-tabs-mode.
(describe-char-after): Maybe use the text property for syntax
table information.  Maybe report char-code-property-table info.
Maybe report character's unicode.  Tweak printing of list info.
(list-input-methods): Add xref buttons.
(dump-charsets, dump-codings): Deleted (obsolete).
From Dave Love <fx@gnu.org>.