bug#55738: character escape bugs in the reader

["Mattias Engdegård" <mattiase@acm.org>]

Some character escape oddities observed in the Emacs reader:

1. ?\LF => -1

This is clearly a bug (no character literal should be -1) and an artefact of the underlying implementation.
The correct value should be 10.
(In string literals \LF is ignored entirely, as documented.)

2. The Control modifier (\C- or \^) is nonidempotent. For example,
?\C-a => 1
?\C-\C-a => #x4000001

Similarly, "\C-\C-a" signals a reader error.

This too is an artefact of the implementation. The correct value should be as if only a single control modifier were present, eg. ?\C-\C-a => 1.

3. Control-space yields NUL in strings but not as a char literal:
"\C-SPC" => "NUL"
"\^SPC"  => "NUL"
?\C-SPC => #x4000020
?\^SPC  => #x4000020

Emacs takes a conservative stance and normally only generates control characters from upper and lower case ASCII letters and the symbols ?@[\]^_ because that agrees with custom and suffices for all C0 controls. Since most terminals also map Control-SPC to NUL, it would be more consistent to do so in both string and character literals.

The first two bugs are straightforward to fix (I have a patch) and doing so is unlikely to cause any harm.
I honestly don't think making ?\C-SPC => 0 would either (because of how key binding words) but we should investigate further just in case.