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/dirread.3

@@ -19,11 +19,11 @@ long dirreadall(int fd, Dir **buf)
 #define	DIRMAX	(sizeof(Dir)+STATMAX)
 .SH DESCRIPTION
 The data returned by a
-.IR read (3)
+.IM read (3)
 on a directory is a set of complete directory entries
 in a machine-independent format, exactly equivalent to
 the result of a
-.IR stat (3)
+.IM stat (3)
 on each file or subdirectory in the directory.
 .I Dirread
 decodes the directory entries into a machine-dependent form.
@@ -35,11 +35,11 @@ structures
 whose address is returned in
 .B *buf
 (see
-.IR stat (3)
+.IM stat (3)
 for the layout of a
 .BR Dir ).
 The array is allocated with
-.IR malloc (3)
+.IM malloc (3)
 each time
 .I dirread
 is called.
@@ -50,7 +50,7 @@ is like
 but reads in the entire directory; by contrast,
 .I dirread
 steps through a directory one
-.IR read (3)
+.IM read (3)
 at a time.
 .PP
 Directory entries have variable length.
@@ -85,9 +85,9 @@ The file offset is advanced by the number of bytes actually read.
 .SH SOURCE
 .B \*9/src/lib9/dirread.c
 .SH SEE ALSO
-.IR intro (3),
-.IR open (3),
-.IR read (3)
+.IM intro (3) ,
+.IM open (3) ,
+.IM read (3)
 .SH DIAGNOSTICS
 .I Dirread
 and