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/man7/plumb.7

@@ -6,7 +6,7 @@ plumb \- format of plumb messages and rules
 .SH DESCRIPTION
 .SS "Message format
 The messages formed by the
-.IR plumb (3)
+.IM plumb (3)
 library are formatted for transmission between
 processes into textual form, using newlines to separate
 the fields.
@@ -61,7 +61,7 @@ A missing field is represented by an empty line.
 The
 .B plumber
 (see
-.IR plumb (1))
+.IM plumb (1) )
 receives messages on its
 .B send
 port (applications
@@ -271,7 +271,7 @@ rule should be specified in a rule set.
 .RE
 .PP
 The arguments to all rules may contain quoted strings, exactly as in
-.IR rc (1).
+.IM rc (1) .
 They may also contain simple string variables, identified by a leading dollar sign
 .BR $ .
 Variables may be set, between rule sets, by assignment statements in the style of
@@ -341,7 +341,7 @@ field of the message.
 .B $plan9
 The root directory of the Plan 9 tree
 (see
-.IR get9root (3)).
+.IM get9root (3) ).
 .RE
 .SH EXAMPLE
 The following is a modest, representative file of plumbing rules.
@@ -403,7 +403,7 @@ default rules file.
 .TP
 .B plumb
 service name for
-.IR plumber (4).
+.IM plumber (4) .
 .TP
 .B \*9/plumb
 directory for
@@ -416,7 +416,7 @@ public macro definitions.
 .B \*9/plumb/basic
 basic rule set.
 .SH "SEE ALSO"
-.IR plumb (1),
-.IR plumb (3),
-.IR plumber (4),
-.IR regexp (7)
+.IM plumb (1) ,
+.IM plumb (3) ,
+.IM plumber (4) ,
+.IM regexp (7)