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/sysfatal.3

@@ -16,17 +16,17 @@ void sysfatal(char *fmt, ...)
 prints to standard error the name of the running program,
 a colon and a space, 
 the message described by the
-.IR print (3)
+.IM print (3)
 format string
 .I fmt
 and subsequent arguments, and a newline.
 It then calls
-.IR exits (3)
+.IM exits (3)
 with the formatted message as argument.
 The program's name is the value of
 .BR argv0 ,
 which will be set if the program uses the
-.IR arg (3)
+.IM arg (3)
 interface to process its arguments.
 If
 .B argv0
@@ -44,7 +44,7 @@ The message is a line with several fields:
 the name of the machine writing the message;
 the date and time;
 the message specified by the 
-.IR print (3)
+.IM print (3)
 format
 .I fmt
 and any following arguments;
@@ -60,9 +60,9 @@ can be used safely in multi-threaded programs.
 .br
 .B \*9/src/lib9/syslog.c
 .SH "SEE ALSO"
-.IR intro (3),
-.IR errstr (3),
+.IM intro (3) ,
+.IM errstr (3) ,
 the
 .B %r
 format in
-.IR print (3)
+.IM print (3)