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, 34 insertions, 0 deletions

diff --git a/regress/roff/esc/invalid.out_ascii b/regress/roff/esc/invalid.out_ascii
new file mode 100644
index 00000000..24915262
--- /dev/null
+++ b/regress/roff/esc/invalid.out_ascii
@@ -0,0 +1,34 @@
+ESC-INVALID(1)              General Commands Manual             ESC-INVALID(1)
+
+
+
+NNAAMMEE
+       esc-invalid - invalid roff escape sequences
+
+DDEESSCCRRIIPPTTIIOONN
+       plus: a+bc
+       semicolon: a;bc
+       less than: a<bc
+       equal to: a=bc
+       greater than: a>bc
+       at: a@bc
+       square bracket: a]b
+       curly braces: abc
+       digit: a1bc
+       G: aGbc
+       I: aIbc
+       i: aibc
+       J: aJbc
+       j: ajbc
+       K: aKbc
+       P: aPbc
+       Q: aQbc
+       q: aqbc
+       T: aTbc
+       U: aUbc
+       W: aWbc
+       y: aybc
+
+
+
+OpenBSD                        December 15, 2018                ESC-INVALID(1)