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/ndb.1

@@ -24,9 +24,9 @@ ndbquery, ndbmkhash, ndbmkdb, ndbipquery, ndbmkhosts \- network database
 The network database holds administrative information used by
 .I authdial
 (see
-.IR authsrv (3))
+.IM authsrv (3) )
 and
-.IR secstored (1).
+.IM secstored (1) .
 .PP
 .I Ndbquery 
 searches the database for an attribute of type
@@ -46,7 +46,7 @@ of all the matched entries is returned.
 uses
 .I ndbipinfo
 (see
-.IR ndb (3))
+.IM ndb (3) )
 to search for the values of the attributes
 .I rattr
 corresponding to the system
@@ -382,7 +382,7 @@ and by the ndb library routines.
 .PP
 .I Ndbmkdb
 is used in concert with
-.IR awk (1)
+.IM awk (1)
 scripts to convert
 uucp systems files and IP host files
 into database files.
@@ -395,7 +395,7 @@ it is necessary to run
 .I ndbmkhash
 whenever the files are modified.
 It may be profitable to control this by a frequent
-.IR cron (8)
+.IM cron (8)
 job.
 .PP
 .I Ndbmkhosts
@@ -439,5 +439,5 @@ hash files for
 .SH SOURCE
 .B \*9/src/cmd/ndb
 .SH SEE ALSO
-.IR ndb (3),
-.IR ndb (7)
+.IM ndb (3) ,
+.IM ndb (7)