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/man4/smugfs.4

@@ -24,7 +24,7 @@ is a user-level file system that provides access to images
 stored on the SmugMug photo sharing service.
 It logs in after
 obtaining a password from 
-.IR factotum (4)
+.IM factotum (4)
 using
 .B server=smugmug.com
 and
@@ -32,7 +32,7 @@ and
 (if any)
 as key criteria
 (see
-.IR auth (3)).
+.IM auth (3) ).
 Then 
 .I smugfs
 serves a virtual directory tree mounted at
@@ -265,7 +265,7 @@ SmugMug,
 If multiple categories or albums have the same name,
 only one will be accessible via the file system interface.
 Renaming the accessible one via Unix's
-.IR mv (1)
+.IM mv (1)
 will resolve the problem.
 .PP
 Boolean values appear as