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/fcall.3

@@ -225,7 +225,7 @@ by a successful call to
 Another structure is
 .BR Dir ,
 used by the routines described in
-.IR stat (3).
+.IM stat (3) .
 .I ConvM2D
 converts the machine-independent form starting at
 .I ap
@@ -293,7 +293,7 @@ contain a validly formatted machine-independent
 entry suitable as an argument, for example, for the
 .B wstat
 (see
-.IR stat (3))
+.IM stat (3) )
 system call.
 It checks that the sizes of all the elements of the the entry sum to exactly
 .IR nbuf ,
@@ -321,7 +321,7 @@ for an incorrectly formatted entry.
 and
 .I dirmodefmt
 are formatting routines, suitable for
-.IR fmtinstall (3).
+.IM fmtinstall (3) .
 They convert
 .BR Dir* ,
 .BR Fcall* ,
@@ -343,7 +343,7 @@ with format letter
 .PP
 .I Read9pmsg
 calls
-.IR read (3)
+.IM read (3)
 multiple times, if necessary, to read an entire 9P message into
 .BR buf .
 The return value is 0 for end of file, or -1 for error; it does not return
@@ -351,7 +351,7 @@ partial messages.
 .SH SOURCE
 .B \*9/src/lib9
 .SH SEE ALSO
-.IR intro (3),
-.IR 9p (3),
-.IR stat (3),
+.IM intro (3) ,
+.IM 9p (3) ,
+.IM stat (3) ,
 .IR intro (9p)