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/man7/venti.7

@@ -14,19 +14,19 @@ of clients.
 This manual page documents the basic concepts of
 block storage using Venti as well as the Venti network protocol.
 .PP
-.IR Venti (1)
+.IM Venti (1)
 documents some simple clients.
-.IR Vac (1),
-.IR vacfs (4),
+.IM Vac (1) ,
+.IM vacfs (4) ,
 and
-.IR vbackup (8)
+.IM vbackup (8)
 are more complex clients.
 .PP
-.IR Venti (3)
+.IM Venti (3)
 describes a C library interface for accessing
 Venti servers and manipulating Venti data structures.
 .PP
-.IR Venti (8)
+.IM Venti (8)
 describes the programs used to run a Venti server.
 .PP
 .SS "Scores
@@ -40,11 +40,11 @@ Scores may have an optional
 prefix, typically used to
 describe the format of the data.
 For example, 
-.IR vac (1)
+.IM vac (1)
 uses a
 .B vac:
 prefix, while
-.IR vbackup (8)
+.IM vbackup (8)
 uses prefixes corresponding to the file system
 types: 
 .BR ext2: ,
@@ -93,7 +93,7 @@ Keeping this parallel representation is a minor annoyance
 but makes it possible for general programs like
 .I venti/copy
 (see
-.IR venti (1))
+.IM venti (1) )
 to traverse the block tree without knowing the specific details
 of any particular program's data.
 .SS "Block Types
@@ -202,7 +202,7 @@ Text strings are represented similarly,
 using a two-byte count with
 the text itself stored as a UTF-encoded sequence
 of Unicode characters (see
-.IR utf (7)).
+.IM utf (7) ).
 Text strings are not
 .SM NUL\c
 -terminated:
@@ -395,7 +395,7 @@ Use
 and
 .I vtfromdisktype
 (see
-.IR venti (3))
+.IM venti (3) )
 to convert a block type enumeration value
 .RB ( VtDataType ,
 etc.)
@@ -457,9 +457,9 @@ in the
 packet may be either 2 or 4 bytes;
 the total packet length distinguishes the two cases.
 .SH SEE ALSO
-.IR venti (1),
-.IR venti (3),
-.IR venti (8)
+.IM venti (1) ,
+.IM venti (3) ,
+.IM venti (8)
 .br
 Sean Quinlan and Sean Dorward,
 ``Venti: a new approach to archival storage'',