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/dict.1

@@ -53,7 +53,7 @@ Print a pronunciation key.
 .PD
 .PP
 Patterns are regular expressions (see
-.IR regexp (7)),
+.IM regexp (7) ),
 with an implicit leading
 .L ^
 and trailing
@@ -154,7 +154,7 @@ searches for dictionaries in the directory named by
 .PP
 .I Adict
 is a dictionary browser for
-.IR acme (1).
+.IM acme (1) .
 When run with no arguments, it creates a new
 .I acme
 window named
@@ -193,7 +193,7 @@ window.
 dictionaries
 .PD
 .SH "SEE ALSO"
-.IR regexp (7)
+.IM regexp (7)
 .SH SOURCE
 .B \*9/src/cmd/dict
 .br