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/ascii.1

@@ -91,7 +91,7 @@ control characters or insert newlines.
 is similar; it converts between
 .SM UTF
 and character values from the Unicode Standard (see
-.IR utf (7)).
+.IM utf (7) ).
 If given a range of hexadecimal numbers,
 .I unicode
 prints a table of the specified Unicode characters \(em their values and
@@ -126,7 +126,7 @@ The file
 contains a
 table of characters and descriptions, sorted in hexadecimal order,
 suitable for
-.IR look (1)
+.IM look (1)
 on the lower case
 .I hex
 values of characters.
@@ -154,7 +154,7 @@ table of characters and descriptions.
 .br
 .B \*9/src/cmd/unicode.c
 .SH "SEE ALSO"
-.IR look (1),
-.IR tcs (1),
-.IR utf (7),
-.IR font (7)
+.IM look (1) ,
+.IM tcs (1) ,
+.IM utf (7) ,
+.IM font (7)