hexedit: various improvements.

I've been using hexedit quite a lot, mainly for _corrupting_ files, and
have been meaning to send this collection of changes for far too long
now. I saw a bug requesting editing in the ASCII pane (which this patch
_doesn't_ add), and wanted to get this sent in before it has to undergo
the third massive merge conflict of its existence...

The main "TODO" in this is that I never got round to implementing
searching for an arbitrary byte sequence. It seems like we ought to have
that feature, but personally I'm far more likely to jump to an offset or
to search for some ASCII. I haven't needed to search for arbitrary byte
sequences in all this time, so I'll fix this if/when I actually need
it...

* Enter (new) read-only mode rather than refusing to open read-only
  files.

* More keys: page up/page down, home/end, and ctrl-home/ctrl-end for
  beginning/end of file.

* Jump with ^J (or vi-like :). Enter absolute address or +12 or -40 for
  relative jumps.

* Find with ^F (or vi-like /). No support for bytes, but useful for
  finding text. (^G or n for next match, ^D or p for previous match.)

* Support all the usual suspects for "quit": vi-like q, desktop-like ^Q,
  panic ^C, or even plain old Esc.

* The ASCII pane is made more readable by (hopefully) reasonable use of
  color. Regular control characters are shown in red using the
  appropriate letter (so a red A is 0x01, etc), printable characters are
  shown normally, and top-bit set characters are just shown as a purple
  question mark (since I couldn't come up with a better representation
  that had any obvious value --- in my experience top-bit set characters
  are either meaningless in ASCII, part of a UTF-8 sequence in modern
  files, or in some random code page in ancient files). The choice of
  red and purple was to deliberately make these not-actually-ASCII
  characters slide into the background; before this patch they have so
  many bright pixels (especially with the use of reverse video) that I
  couldn't clearly see the *actual* ASCII content in the ASCII pane.

* Addresses are now shown in yellow. No real justification other than "it
  looks nice".

* NUL bytes in the hex pane are shown dimmed. I find this helpful
  especially when there's a lot of padding, and it can actually be a
  useful clue when reverse engineering (you can "see" repeated patterns
  more easily), but I can understand if this one's controversial.

* Errors are shown "vim style" in bold white text on a red background,
  waiting briefly to ensure they're seen.

* The status bar shows the filename, whether the file is opened
  read-only, the current offset into the file, and the total
  length of the file.

* SIGWINCH handling has been added.

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