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/exec.3

@@ -25,11 +25,11 @@ points to the name of the file
 to be executed; it must not be a directory, and the permissions
 must allow the current user to execute it
 (see
-.IR stat (3)).
+.IM stat (3) ).
 It should also be a valid binary image, as defined by the local
 operating system, or a shell script
 (see
-.IR rc (1)).
+.IM rc (1) ).
 The first line of a
 shell script must begin with
 .L #!
@@ -92,24 +92,24 @@ files remain open across
 .B OCEXEC
 OR'd
 into the open mode; see
-.IR open (3));
+.IM open (3) );
 and the working directory and environment
 (see
-.IR getenv (3))
+.IM getenv (3) )
 remain the same.
 However, a newly
 .I exec'ed
 process has no notification handlers
 (see
-.IR notify (3)).
+.IM notify (3) ).
 .SH SOURCE
 .B \*9/src/lib9/exec.c
 .br
 .B \*9/src/lib9/execl.c
 .SH SEE ALSO
-.IR prof (1),
-.IR intro (3),
-.IR stat (3)
+.IM prof (1) ,
+.IM intro (3) ,
+.IM stat (3)
 .SH DIAGNOSTICS
 If these functions fail, they return and set
 .IR errstr .
@@ -138,4 +138,4 @@ are preprocessor macros defined as
 and
 .IR p9execl ;
 see
-.IR intro (3).
+.IM intro (3) .