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/ndb.3

@@ -88,13 +88,13 @@ Ndbtuple*	ndbsubstitute(Ndbtuple *t, Ndbtuple *from, Ndbtuple *to);
 These routines are used by network administrative programs to search
 the network database.
 They operate on the database files described in
-.IR ndb (7).
+.IM ndb (7) .
 .PP
 .I Ndbopen
 opens the database
 .I file
 and calls
-.IR malloc (3)
+.IM malloc (3)
 to allocate a buffer for it.
 If
 .I file
@@ -128,7 +128,7 @@ is used to find each successive match.
 On a successful search both return a linked list of
 .I Ndbtuple
 structures acquired by
-.IR malloc (3)
+.IM malloc (3)
 that represent the attribute/value pairs in the
 entry.
 On failure they return zero.
@@ -450,8 +450,8 @@ directory of network database files
 .SH SOURCE
 .B \*9/src/libndb
 .SH SEE ALSO
-.IR ndb (1)
-.IR ndb (7)
+.IM ndb (1)
+.IM ndb (7)
 .SH DIAGNOSTICS
 .IR Ndbgetvalue
 and