Relax fatal circumstances of unescaped '{'

Relax fatal circumstances of unescaped '{'

After the 5.26.0 code freeze, it came out that an application that many
others depend on, GNU Autoconf, has an unescaped '{' in it.  Commit
7335cb814c19345052a23bc4462c701ce734e6c5 created a kludge that was
minimal, and designed to get just that one application to work.

I originally proposed a less kludgy patch that was applicable across a
larger set of applications.  The proposed patch didn't fatalize uses
of unesacped '{' where we don't anticipate using it for something other
than its literal self.  That approach worked for Autoconf, but also far
more instances, but was more complicated, and was rejected as being too
risky during code freeze.

Now this commit implements my original suggestion.  I am putting it in
now, to let it soak in blead, in case something else surfaces besides
Autoconf, that we need to work around.  By having experience with the
patch live, we can be more confident about using it, if necessary, in a
dot release.