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/man1/9.1

@@ -12,7 +12,7 @@
 .B .
 .B 9
 (from
-.IR sh (1))
+.IM sh (1) )
 .PP
 .B 9.rc
 .I cmd
@@ -24,7 +24,7 @@
 .B .
 .B 9.rc
 (from
-.IR rc (1))
+.IM rc (1) )
 .PP
 .B u
 .I cmd
@@ -36,7 +36,7 @@
 .B .
 .B u
 (from
-.IR sh (1))
+.IM sh (1) )
 .PP
 .B u.rc
 .I cmd
@@ -48,7 +48,7 @@
 .B .
 .B u.rc
 (from
-.IR rc (1))
+.IM rc (1) )
 .SH DESCRIPTION
 Because Plan 9 supplies commands with the same name as but different
 behavior than many basic Unix system commands
@@ -86,7 +86,7 @@ in order to make the current shell start running in the Plan 9 environment.
 is the same as
 .I 9
 but written for use by the shell
-.IR rc (1).
+.IM rc (1) .
 .PP
 .I U
 and
@@ -106,7 +106,7 @@ $ 9 grep '[α-ζ]' /etc/passwd
 .EE
 .PP
 Start an
-.IR rc (1)
+.IM rc (1)
 with the Plan 9 commands in the path before the system commands,
 and then run the Unix
 .IR ls :
@@ -124,7 +124,7 @@ $ 9 rc
 .br
 .B \*9/bin/u.rc
 .SH SEE ALSO
-.IR intro (1)
+.IM intro (1)
 .SH BUGS
 Some shell configurations
 (notably, oh-my-zsh)