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/rsa.1

@@ -85,7 +85,7 @@ Plan 9 represents DSA and RSA keys as attribute-value pair lists
 prefixed with the string
 .BR key ;
 this is the generic key format used by
-.IR factotum (4).
+.IM factotum (4) .
 A full DSA private key has the following attributes:
 .TP
 .B proto
@@ -334,9 +334,9 @@ scp auth.keys unix:.ssh/authorized_keys
 .SH SOURCE
 .B \*9/src/cmd/auth
 .SH "SEE ALSO
-.IR factotum (4),
-.IR pem (1),
-.IR ssh (1)
+.IM factotum (4) ,
+.IM pem (1) ,
+.IM ssh (1)
 .SH BUGS
 There are too many key formats.
 .PP