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/get9root.3

@@ -49,7 +49,7 @@ if different from
 should be freed with
 .I free
 (see
-.IR malloc (3))
+.IM malloc (3) )
 when no longer needed.
 .PP
 As a convention, programs should never
@@ -57,7 +57,7 @@ As a convention, programs should never
 paths obtained from user input.
 .SH EXAMPLE
 The
-.IR plumber (4)
+.IM plumber (4)
 uses this code to find unrooted file names included by plumb rules.
 .IP
 .EX
@@ -69,7 +69,7 @@ fd = open(unsharp(buf), OREAD);
 .br
 .B \*9/src/lib9/unsharp.c
 .SH SEE ALSO
-.IR intro (4)
+.IM intro (4)
 .SH BUGS
 .I Get9root
 could be smarter about finding the tree when