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/wait.3

@@ -29,9 +29,9 @@ int		awaitfor(int pid, char *s, int n)
 .SH DESCRIPTION
 .I Wait
 causes a process to wait for any child process (see
-.IR fork (2)
+.IM fork (2)
 and
-.IR rfork (3))
+.IM rfork (3) )
 to exit.
 It returns a
 .B Waitmsg
@@ -62,7 +62,7 @@ the time spent in system calls, and the child's elapsed real time,
 all in units of milliseconds.
 .B Msg
 contains the message that the child specified in
-.IR exits (3).
+.IM exits (3) .
 For a normal exit,
 .B msg[0]
 is zero,
@@ -78,7 +78,7 @@ returns immediately, with return value nil.
 The
 .B Waitmsg
 structure is allocated by
-.IR malloc (3)
+.IM malloc (3)
 and should be freed after use.
 For programs that only need the pid of the exiting program,
 .I waitpid
@@ -114,7 +114,7 @@ The filled-in buffer
 may be parsed (after appending a NUL) using
 .IR tokenize
 (see
-.IR getfields (3));
+.IM getfields (3) );
 the resulting fields are, in order, pid, the three times, and the exit string,
 which will be
 .B ''
@@ -139,8 +139,8 @@ returns
 .PP
 .B \*9/src/lib9/await.c
 .SH "SEE ALSO"
-.IR rfork (3),
-.IR exits (3),
+.IM rfork (3) ,
+.IM exits (3) ,
 .SH DIAGNOSTICS
 These routines set
 .IR errstr .
@@ -156,4 +156,4 @@ are preprocessor macros defined as
 and
 .IR p9waitfor ;
 see 
-.IR intro (3).
+.IM intro (3) .