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/sam.1

@@ -51,7 +51,7 @@ copies leading white space on the current line to the new line.
 Do not `download' the terminal part of
 .IR sam .
 Editing will be done with the command language only, as in
-.IR ed (1).
+.IM ed (1) .
 .TP
 .BI -r " machine
 Run the host part remotely
@@ -69,7 +69,7 @@ for debugging.
 .PD
 .SS Regular expressions
 Regular expressions are as in
-.IR regexp (7)
+.IM regexp (7)
 with the addition of
 .BR \en
 to represent newlines.
@@ -698,7 +698,7 @@ There is usually a `current window',
 marked with a dark border, to which typed text and editing
 commands apply.
 Text may be typed and edited as in
-.IR rio (1);
+.IM rio (1) ;
 also the escape key (ESC) selects (sets dot to) text typed
 since the last mouse button hit.
 .PP
@@ -798,7 +798,7 @@ the white-space-delimited block of text is sent as a plumb message
 with a
 .B click
 attribute defining where the selection lies (see
-.IR plumb (7)).
+.IM plumb (7) ).
 .TP
 .B look
 Search forward for the next occurrence of the literal text in dot.
@@ -909,11 +909,11 @@ source for the separate terminal part
 .TP
 .B \*9/bin/E
 .SH SEE ALSO
-.IR ed (1),
-.IR sed (1),
-.IR grep (1),
-.IR rio (1),
-.IR regexp (7).
+.IM ed (1) ,
+.IM sed (1) ,
+.IM grep (1) ,
+.IM rio (1) ,
+.IM regexp (7) .
 .PP
 Rob Pike,
 ``The text editor sam''.