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/mk9660.1

@@ -71,11 +71,11 @@ the current directory).
 The
 .I proto
 file is formatted as described in
-.IR proto (3).
+.IM proto (3) .
 .PP
 The created CD image will be in ISO-9660
 format, but by default the file names will
-be stored in UTF-8 with no imposed length 
+be stored in UTF-8 with no imposed length
 or character restrictions.
 The
 .B -c
@@ -84,7 +84,7 @@ flag causes
 to use only file names in ``8.3'' form
 that use digits, letters, and underscore.
 File names that do not conform are changed
-to 
+to
 .BI D nnnnnn
 (for directories)
 or
@@ -122,7 +122,7 @@ and
 .B \e
 are allowed in Plan 9 file names but not in Joliet file names;
 non-conforming file names are translated
-and a 
+and a
 .B _CONFORM.MAP
 file written
 as in the case of the
@@ -133,7 +133,7 @@ If the
 .B -r
 flag is given, Rock Ridge extensions are written in the
 format of the system use sharing protocol;
-this format provides Posix-style file metadata and is 
+this format provides Posix-style file metadata and is
 common on Unix platforms.
 .PP
 The options
@@ -155,13 +155,13 @@ if unspecified, the base name of
 .I proto
 is used.
 .PP
-The 
+The
 .B -:
-flag causes 
+flag causes
 .B mk9660
 to replace colons in scanned file names with spaces;
 this is the inverse of the map applied by Plan 9's
-\fIdossrv\fR(4)
+.IR dossrv (4)
 and is useful for writing Joliet CDs containing data
 from FAT file systems.
 .PP
@@ -189,10 +189,11 @@ on standard error.
 is similar in specification to
 .I mk9660
 but creates and updates backup CD images in the style of
-the 
+the
 .I dump
 file system
-(see Plan 9's \fIfs\fR(4)).
+(see Plan 9's
+.IR fs (4)).
 The dump is file-based rather than block-based:
 if a file's contents have not changed since the last
 backup, only its directory entry will be rewritten.
@@ -202,7 +203,7 @@ The
 option specifies a time (in seconds since January 1, 1970)
 to be used for naming the dump directory.
 .PP
-The 
+The
 .B -m
 option specifies a maximum size for the image;
 if a backup would cause the image to grow larger than
@@ -212,7 +213,7 @@ it will not be written, and
 will exit with a non-empty status.
 .SH EXAMPLE
 .PP
-Create an image of the Plan 9 source tree, 
+Create an image of the Plan 9 source tree,
 including a conformant ISO-9660 directory tree,
 Plan 9 extensions in the system use fields, and
 a Joliet directory tree.
@@ -223,7 +224,7 @@ mk9660 -9cj -s /n/bootes -p srcproto cdimage
 .SH SOURCE
 \*9/src/cmd/9660
 .SH "SEE ALSO
-.IR proto (3)
+.IM proto (3)
 .\" .SH "SEE ALSO"
 .\" .I 9660srv
 .\" (in