obscure new display features

[Dave Love]

I find the new display features concerning `escape-glyph' and
`show-nonbreak-escape' annoying, but also they're obscure and partly
inconsistent.  This is roughly what I went through on encountering
them.

First you wonder why, say, ^L in your source files is apparently being
font-locked as a keyword.  You use C-u C-x =, and there's no clue --
no face property or display table entry.  You then look for an overlay
with no joy.  Huh?  Surely something like this should be done with a
display table.

You have no clue how to turn it off without finding the entry in NEWS.
There apparently isn't anything in Custom about it, either in the
`display' group or via M-x customize-changed.  You need to know to
look for it either in a face doc string or the Lisp manual, but I
wouldn't guess to search for `escape-glyph'.

The situation is similar for `show-nonbreak-escape'.  Actually that's
more annoying since it changes the display and layout of things likely
actually to use NBSP for layout.  It's also inconsistent and partly
incorrect.  I've no idea why non-breaking characters should be
displayed like this, but U+00AD isn't one -- it's SOFT HYPHEN.  If
you're going to change its display, the issue (see Unicode) is whether
or not it should be displayed at all -- not that I think it should be
invisible.  NON-BREAKING HYPHEN is U+2011.  The inconsistency is that
non-breaking space is at codepoint 32 in at least all the iso8859
Emacs charsets, but apparently only the 8859-1 version is treated.