Since \. is not a character escape sequence, re-classify it from the

Since \. is not a character escape sequence, re-classify it from the
wrong parsing class ESCAPE_SPECIAL to the better-suited parsing class
ESCAPE_UNDEF, exactly like it is already done for the similar \\,
which isn't a character escape sequence either.

No formatting change is intended just yet, but this will matter for
upcoming improvements in the parser for roff(7) macro, string, and
register names.

See the node "5.23.2 Copy Mode" in "info groff" regarding
what \\ and \. really mean.