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/man8/venti.8

@@ -40,7 +40,7 @@ venti \- archival storage server
 .I Venti
 is a SHA1-addressed archival storage server.
 See 
-.IR venti (7)
+.IM venti (7)
 for a full introduction to the system.
 This page documents the structure and operation of the server.
 .PP
@@ -138,7 +138,7 @@ less than 10 are not very useful;
 greater than 24 are probably a waste of memory.
 .I Fmtbloom
 (see
-.IR venti-fmt (8))
+.IM venti-fmt (8) )
 can be given either
 .I nhash
 or
@@ -220,7 +220,7 @@ The venti server announces two network services, one
 .BR venti ,
 17034) serving
 the venti protocol as described in
-.IR venti (7),
+.IM venti (7) ,
 and one serving HTTP
 (conventionally TCP port 
 .BR http ,
@@ -333,7 +333,7 @@ with
 or 
 .I fmtisect
 (see
-.IR venti-fmt (8)).
+.IM venti-fmt (8) ).
 In particular, only the configuration needs to be
 changed if a component is moved to a different file.
 .PP
@@ -506,11 +506,11 @@ Start the server and check the storage statistics:
 .SH SOURCE
 .B \*9/src/cmd/venti/srv
 .SH "SEE ALSO"
-.IR venti (1),
-.IR venti (3),
-.IR venti (7),
-.IR venti-backup (8)
-.IR venti-fmt (8)
+.IM venti (1) ,
+.IM venti (3) ,
+.IM venti (7) ,
+.IM venti-backup (8)
+.IM venti-fmt (8)
 .br
 Sean Quinlan and Sean Dorward,
 ``Venti: a new approach to archival storage'',
@@ -523,7 +523,7 @@ Venti should not require the user to decide how to
 partition its memory usage.
 .PP
 Users of shells other than 
-.IR rc (1)
+.IM rc (1)
 will not be able to use the program names shown.
 One solution is to define
 .B "V=$PLAN9/bin/venti"