Several improvements to escape sequence handling.

* Add the missing special character \_ (underscore).
* Partial implementations of \a (leader character)
and \E (uninterpreted escape character).
* Parse and ignore \r (reverse line feed).
* Add a WARNING message about undefined escape sequences.
* Add an UNSUPP message about unsupported escape sequences.
* Mark \! and \? (transparent throughput)
and \O (suppress output) as unsupported.
* Treat the various variants of zero-width spaces as one-byte escape
sequences rather than as special characters, to avoid defining bogus
forms with square brackets.
* For special characters with one-byte names, do not define bogus
forms with square brackets, except for \[-], which is valid.
* In the form with square brackets, undefined special characters do not
fall back to printing the name verbatim, not even for one-byte names.
* Starting a special character name with a blank is an error.
* Undefined escape sequences never abort formatting of the input
string, not even in HTML output mode.
* Document the newly handled escapes, and a few that were missing.
* Regression tests for most of the above.

1 files changed, 11 insertions, 7 deletions

diff --git a/regress/roff/esc/one.out_ascii b/regress/roff/esc/one.out_ascii
index bc88dcf2..1eb5f1f2 100644
--- a/regress/roff/esc/one.out_ascii
+++ b/regress/roff/esc/one.out_ascii
@@ -1,13 +1,17 @@
 ESC-ONE(1)                  General Commands Manual                 ESC-ONE(1)
 
+
+
 NNAAMMEE
-     eesscc--oonnee - roff one-character escape sequences
+       esc-one - roff one-character escape sequences
 
 DDEESSCCRRIIPPTTIIOONN
-     backslash: >\<
-     minus: >-<
-     acute: >'<
-     grave: >`<
-     normal character: >q<
+       backslash: >\<
+       minus: >-|-<
+       acute: >'<
+       grave: >`<
+       normal character: >q<
+
+
 
-OpenBSD                          July 4, 2017                          OpenBSD
+OpenBSD                        December 15, 2018                    ESC-ONE(1)