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/acid.1

@@ -70,7 +70,7 @@ at startup; see below.
 .BI -m " machine
 Assume instructions are for the given CPU type
 (see
-.IR mach (3))
+.IM mach (3) )
 instead of using the executable header to select
 the CPU type.
 .TP
@@ -145,7 +145,7 @@ subscripts counted from 0.
 .BI delete " list", " subscript
 .PP
 Format codes are the same as in
-.IR db (1).
+.IM db (1) .
 Formats may be attached to (unary) expressions with
 .BR \e ,
 e.g.
@@ -230,7 +230,7 @@ Print 10 lines of source around the program address.
 .BI Bsrc( address )
 Get the source line for the program address
 into a window of a running
-.IR sam (1)
+.IM sam (1)
 and select it.
 .TP
 .BI line( address )
@@ -323,7 +323,7 @@ Make the given process current.
 .TP
 .BI rc( string )
 Escape to the shell,
-.IR rc (1),
+.IM rc (1) ,
 to execute the command string.
 .TP
 .BI include( string )
@@ -449,7 +449,7 @@ notation)
 .BR *array .
 .PP
 Trace the system calls executed by
-.IR ls (1)
+.IM ls (1)
 (neither does this one):
 .IP
 .EX
@@ -503,8 +503,8 @@ acid: cont()
 .SH SOURCE
 .B \*9/src/cmd/acid
 .SH "SEE ALSO"
-.IR mk (1),
-.IR db (1)
+.IM mk (1) ,
+.IM db (1)
 .br
 Phil Winterbottom,
 ``Acid Manual''.