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/ls.1

@@ -29,7 +29,7 @@ is the same as
 but sets the
 .B -p
 option and pipes the output through
-.IR mc (1).
+.IM mc (1) .
 .PP
 There are a number of options:
 .TP
@@ -42,7 +42,7 @@ List in long format, giving mode (see below), file system type
 (e.g., for devices, the
 .B #
 code letter that names it; see
-.IR intro (3)),
+.IM intro (3) ),
 the instance or subdevice number, owner, group,
 size in bytes, and time of last modification
 for each file.
@@ -60,7 +60,7 @@ Print only the final path element of each file name.
 List the
 .I qid
 (see
-.IR stat (3))
+.IM stat (3) )
 of each file; the printed fields are in the order
 path, version, and type.
 .TP
@@ -99,7 +99,7 @@ otherwise.
 .TP
 .B -Q
 By default, printed file names are quoted if they contain characters special to
-.IR rc (1).
+.IM rc (1) .
 The
 .B -Q
 flag disables this behavior.
@@ -168,5 +168,5 @@ if none of the above permissions is granted.
 .br
 .B \*9/bin/lc
 .SH SEE ALSO
-.IR stat (3),
-.IR mc (1)
+.IM stat (3) ,
+.IM mc (1)