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/db.1

@@ -45,11 +45,11 @@ specifies the memory image of a process.
 A 
 .I pid
 gives the id of an executing process to be accessed via
-.IR ptrace (2).
+.IM ptrace (2) .
 A
 .I corefile
 specifies the name of a core dump (see
-.IR core (5)
+.IM core (5)
 on your system of choice) containing the
 memory image of a terminated process.
 This manual refers to the memory image specified by
@@ -628,7 +628,7 @@ Dot is assigned to the variable or register named.
 .TP
 .B !
 The rest of the line is passed to
-.IR rc (1)
+.IM rc (1)
 for execution.
 .TP
 .BI $ modifier
@@ -969,8 +969,8 @@ is one the breakpoint will fire.
 Beware that local variables may be stored in registers; see the
 BUGS section.
 .SH "SEE ALSO"
-.IR acid (1),
-.IR core (1)
+.IM acid (1) ,
+.IM core (1)
 .SH SOURCE
 .B \*9/src/cmd/db
 .SH DIAGNOSTICS