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/kill.1

@@ -19,18 +19,18 @@ prints commands that will cause all processes with
 .I name
 and owned by the current user to be terminated.
 Each command is commented with an output line from
-.IR ps (1)
+.IM ps (1)
 describing the process that would be killed.
 Use the
 .B send
 command of
-.IR 9term (1),
+.IM 9term (1) ,
 or pipe the output of
 .I kill
 into
-.IR rc (1)
+.IM rc (1)
 or
-.IR sh (1)
+.IM sh (1)
 to execute the commands.
 .PP
 .I Kill
@@ -60,8 +60,8 @@ signal.
 .SH SOURCE
 .B \*9/bin
 .SH "SEE ALSO"
-.IR ps (1),
-.IR notify (3)
+.IM ps (1) ,
+.IM notify (3)
 .SH BUGS
 .I Stop
 and