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/src.1

@@ -19,11 +19,11 @@ examines the named
 to find the corresponding source code, which is then sent to the editor using
 .B B
 (see
-.IR sam (1)).
+.IM sam (1) ).
 If
 .I file
 is an
-.IR rc (1)
+.IM rc (1)
 script, the source is the file itself.
 If
 .I file
@@ -35,7 +35,7 @@ and
 will point the editor at the line that begins the definition.
 .I Src
 uses
-.IR db (1)
+.IM db (1)
 to extract the symbol table information that identifies the source.
 .PP
 .I Src
@@ -78,6 +78,6 @@ src -s strcmp rc
 .SH SOURCE
 .B \*9/bin/src
 .SH "SEE ALSO"
-.IR db (1),
-.IR plumb (1),
-.IR sam (1).
+.IM db (1) ,
+.IM plumb (1) ,
+.IM sam (1) .