tmac: introduce real manual reference macro instead of overloading IR

The overloading of IR emits magic \X'...' sequences that turn into HTML manual links.
But not all such IR invocations should be manual links;
those had to be written to avoid the IR macro before.
Worse, the \X'...' ending the IR causes troff to emit only a single space after a period.

Defining a new IM macro for manual references fixes both problems.

Fixes #441.

man/man1/yacc.1

@@ -23,7 +23,7 @@ to produce a program
 This program must be loaded with a lexical analyzer function,
 .B yylex(void)
 (often generated by
-.IR lex (1)),
+.IM lex (1) ),
 with a
 .B main(int argc, char *argv[])
 program, and with an error handling routine,
@@ -115,7 +115,7 @@ option reverses this.
 The parser accepts
 .SM UTF
 input text (see
-.IR utf (7)),
+.IM utf (7) ),
 which has a couple of effects.
 First, the return value of
 .B yylex()
@@ -156,7 +156,7 @@ parser prototype using stdio
 .SH SOURCE
 .B \*9/src/cmd/yacc.c
 .SH "SEE ALSO"
-.IR lex (1)
+.IM lex (1)
 .br
 S. C. Johnson and R. Sethi,
 ``Yacc: A parser generator'',