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/man1/label.1

@@ -14,12 +14,12 @@ label, awd \- set window label
 sets the label of the current 
 .I win
 (see
-.IR acme (1))
+.IM acme (1) )
 or X terminal window
 .RI ( e.g.,
-.IR 9term (1)
+.IM 9term (1)
 or
-.IR xterm (1))
+.IM xterm (1) )
 by echoing a special control sequence to standard output.
 .PP
 .I Acme
@@ -38,7 +38,7 @@ sets the window name to the current directory with a
 suffix, using the name of the current system by default.
 .SH EXAMPLE
 One can use the following
-.IR sh (1)
+.IM sh (1)
 function to keep the label up-to-date in response to
 .I cd
 commands:
@@ -55,7 +55,7 @@ alias cd=_cd
 cd .
 .EE
 .PP
-.IR Rc (1)
+.IM Rc (1)
 installs a similar 
 .B fn
 .B cd
@@ -77,4 +77,4 @@ fn cd {
 .SH BUGS
 .I Awd
 is also documented in
-.IR acme (1).
+.IM acme (1) .