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, 3 insertions, 2 deletions

diff --git a/regress/roff/esc/Makefile b/regress/roff/esc/Makefile
index 32589479..74f153d5 100644
--- a/regress/roff/esc/Makefile
+++ b/regress/roff/esc/Makefile
@@ -1,6 +1,7 @@
 # $OpenBSD: Makefile,v 1.11 2015/04/29 18:32:57 schwarze Exp $
 
-REGRESS_TARGETS = one two multi B c c_man e f h l o p w z ignore
-LINT_TARGETS	= B h l w ignore
+REGRESS_TARGETS	 = one two multi B c c_man e f h l O o p w z
+REGRESS_TARGETS	+= ignore invalid unsupp
+LINT_TARGETS	 = B h l O w ignore invalid unsupp
 
 .include <bsd.regress.mk>