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/man3/proto.3

@@ -24,7 +24,7 @@ int rdproto(char *proto, char *root, Protoenum *enm,
 .I Rdproto
 reads and interprets the named
 .I proto
-file relative to the 
+file relative to the
 root directory
 .IR root .
 .PP
@@ -45,9 +45,9 @@ The fifth field is the name of the file from which to copy;
 this file is read from the current name space,
 not the source file tree.
 All fields except the first are optional.
-Specifying 
+Specifying
 .B -
-for permissions, owner, or group 
+for permissions, owner, or group
 causes
 .I rdproto
 to fetch the corresponding information
@@ -92,29 +92,29 @@ Only the
 and
 .B length
 fields are guaranteed to be valid.
-The argument 
+The argument
 .I a
 is the same argument passed to
 .IR rdproto ;
 typically it points at some extra state
 used by the enumeration function.
 .PP
-When files or directories do not exist or 
-cannot be read by 
+When files or directories do not exist or
+cannot be read by
 .IR rdproto ,
-it formats a warning message, calls 
+it formats a warning message, calls
 .IR warn ,
-and continues processing; 
+and continues processing;
 if
 .I warn
-is nil, 
+is nil,
 .I rdproto
 prints the warning message to standard error.
 .PP
 .I Rdproto
 returns zero
 if
-.I proto 
+.I proto
 was processed, \-1 if it could not be opened.
 .SH FILES
 .TF /sys/lib/sysconfig/proto/portproto
@@ -127,5 +127,6 @@ generic prototype file.
 .SH SOURCE
 .B \*9/src/libdisk/proto.c
 .SH SEE ALSO
-.IR mk9660 (1),
-Plan 9's \fImkfs\fR(8)
+.IM mk9660 (1) ,
+Plan 9's
+.IR mkfs (8)