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/man3/print.3

@@ -67,7 +67,7 @@ writes to the named output
 file descriptor:
 a buffered form
 is described in
-.IR bio (3).
+.IM bio (3) .
 .I Sprint
 places text
 followed by the NUL character
@@ -104,7 +104,7 @@ is like
 .IR sprint ,
 except that it prints into and returns a string of the required length, which is
 allocated by
-.IR malloc (3).
+.IM malloc (3) .
 .PP
 The routines
 .IR runesprint ,
@@ -361,7 +361,7 @@ The
 .B S
 verb is similar, but it interprets its pointer as an array
 of runes (see
-.IR utf (7));
+.IM utf (7) );
 the runes are converted to
 .SM UTF
 before output.
@@ -389,10 +389,10 @@ but that will change if pointers and integers are different sizes.
 The
 .B r
 verb takes no arguments; it copies the error string returned by a call to
-.IR errstr (3).
+.IM errstr (3) .
 .PP
 Custom verbs may be installed using
-.IR fmtinstall (3).
+.IM fmtinstall (3) .
 .SH EXAMPLE
 This function prints an error message with a variable
 number of arguments and then quits.
@@ -415,9 +415,9 @@ void fatal(char *msg, ...)
 .SH SOURCE
 .B \*9/src/lib9/fmt
 .SH SEE ALSO
-.IR fmtinstall (3),
-.IR fprintf (3),
-.IR utf (7)
+.IM fmtinstall (3) ,
+.IM fprintf (3) ,
+.IM utf (7)
 .SH DIAGNOSTICS
 Routines that write to a file descriptor or call
 .IR malloc
@@ -425,7 +425,7 @@ set
 .IR errstr .
 .SH BUGS
 The formatting is close to that specified for ANSI
-.IR fprintf (3);
+.IM fprintf (3) ;
 the main difference is that
 .B b
 and