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/rc.1

@@ -50,7 +50,7 @@ exits or is terminated, the
 variable
 .B $status
 gets the process's wait message (see
-.IR wait (3));
+.IM wait (3) );
 it will be the null string if the command was successful.
 .PP
 A long command line may be continued on subsequent lines by typing
@@ -83,7 +83,7 @@ in a directory in
 .B $path
 is the program to be executed.
 To be executable, the user must have execute permission (see
-.IR stat (3))
+.IM stat (3) )
 and the file must be either an executable binary
 for the current machine's CPU type, or a shell script.
 Shell scripts begin with a line containing the full path name of a shell
@@ -350,7 +350,7 @@ or
 is a previously opened file descriptor and
 .I fd0
 becomes a new copy (in the sense of
-.IR dup (3))
+.IM dup (3) )
 of it.
 A file descriptor may be closed by writing
 .BI >[ fd0 =]
@@ -543,7 +543,7 @@ function definition.
 A function with a special name will be called when
 .I rc
 receives a corresponding note; see
-.IR notify (3).
+.IM notify (3) .
 The valid note names (and corresponding notes) are
 .B sighup
 .RB ( hangup ),
@@ -662,7 +662,7 @@ is composed of the bitwise OR of the
 .B rfork
 flags specified by the option letters
 (see
-.IR fork (2)).
+.IM fork (2) ).
 If no
 .I flags
 are given, they default to
@@ -829,7 +829,7 @@ parsing the
 .B $PATH
 variable
 (as in
-.IR sh (1))
+.IM sh (1) )
 or by
 .BR "path=(.\ /bin)" .
 The variables