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/sed.1

@@ -78,7 +78,7 @@ that
 addresses the last line of input, or a context address,
 .BI / regular-expression / \f1,
 in the style of
-.IR regexp (7),
+.IM regexp (7) ,
 with the added convention that
 .L \en
 matches a
@@ -235,7 +235,7 @@ in the pattern space.
 Any character may be used instead of 
 .LR / .
 For a fuller description see
-.IR regexp (7).
+.IM regexp (7) .
 .I Flags
 is zero or more of
 .RS
@@ -374,8 +374,8 @@ formatted manuscript.
 .IR grep (1), 
 .IR awk (1), 
 .IR lex (1), 
-.IR sam (1),
-.IR regexp (7)
+.IM sam (1) ,
+.IM regexp (7)
 .br
 L. E. McMahon,
 `SED \(em A Non-interactive Text Editor',