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/bc.1

@@ -218,7 +218,7 @@ Assignment to
 .B scale
 influences the number of digits to be retained on arithmetic
 operations in the manner of
-.IR dc (1).
+.IM dc (1) .
 Assignments to
 .B ibase
 or
@@ -235,7 +235,7 @@ empty square brackets must follow the array name.
 .PP
 .I Bc
 is actually a preprocessor for
-.IR dc (1),
+.IM dc (1) ,
 which it invokes automatically, unless the
 .B -c
 (compile only)
@@ -273,7 +273,7 @@ mathematical library
 .B \*9/src/cmd/bc.y
 .SH "SEE ALSO"
 .IR dc (1), 
-.IR hoc (1)
+.IM hoc (1)
 .SH BUGS
 No
 .LR && ,