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/man3/frame.3

@@ -73,9 +73,9 @@ enum{
 This library supports
 .I frames
 of editable text in a single font on raster displays, such as in
-.IR sam (1)
+.IM sam (1)
 and
-.IR 9term (1).
+.IM 9term (1) .
 Frames may hold any character except NUL (0).
 Long lines are folded and tabs are at fixed intervals.
 .PP
@@ -239,7 +239,7 @@ If a
 .B Frame
 is being moved but not resized, that is, if the shape of its containing
 rectangle is unchanged, it is sufficient to use
-.IR draw (3)
+.IM draw (3)
 to copy the containing rectangle from the old to the new location and then call
 .I frsetrects
 to establish the new geometry.
@@ -357,6 +357,6 @@ and
 .SH SOURCE
 .B \*9/src/libframe
 .SH SEE ALSO
-.IR graphics (3),
-.IR draw (3),
-.IR cachechars (3).
+.IM graphics (3) ,
+.IM draw (3) ,
+.IM cachechars (3) .