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, 9 insertions, 0 deletions

diff --git a/regress/char/space/invalid.out_lint b/regress/char/space/invalid.out_lint
new file mode 100644
index 00000000..c05ef38f
--- /dev/null
+++ b/regress/char/space/invalid.out_lint
@@ -0,0 +1,9 @@
+mandoc: invalid.in:7:15: WARNING: invalid escape sequence: \[ 
+mandoc: invalid.in:8:14: WARNING: invalid escape sequence: \[%]
+mandoc: invalid.in:9:16: WARNING: invalid escape sequence: \[&]
+mandoc: invalid.in:10:12: WARNING: invalid escape sequence: \[:]
+mandoc: invalid.in:11:12: WARNING: invalid escape sequence: \[^]
+mandoc: invalid.in:12:16: WARNING: invalid escape sequence: \[_]
+mandoc: invalid.in:13:11: WARNING: invalid escape sequence: \[|]
+mandoc: invalid.in:14:12: WARNING: invalid escape sequence: \[~]
+mandoc: invalid.in:15:18: WARNING: invalid escape sequence: \[0]