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/wintext.1

@@ -22,11 +22,11 @@ wintext, ", "" \- access text in current window
 prints the text of the current
 .I win
 (see
-.IR acme (1)),
-.IR 9term (1),
+.IM acme (1) ),
+.IM 9term (1) ,
 or
 (Unix's)
-.IR tmux (1)
+.IM tmux (1)
 window to standard output.
 .PP
 .I \*y
@@ -43,7 +43,7 @@ prints the last command executed.
 prints the last command that
 .I \*y
 would print and then executes it by piping it into
-.IR rc (1).
+.IM rc (1) .
 .PP
 Both
 .I \*y
@@ -61,7 +61,7 @@ or
 .BR # .
 .SH EXAMPLES
 Print the
-.IR ls (1)
+.IM ls (1)
 and
 .I lc
 commands executed in this window:
@@ -87,8 +87,8 @@ ramfs   rc      read    rio     rm
 %
 .EE
 .SH SEE ALSO
-.IR 9term (1),
-.IR acme (1)
+.IM 9term (1) ,
+.IM acme (1)
 .SH SOURCE
 .B \*9/bin
 .SH BUGS
@@ -96,5 +96,5 @@ ramfs   rc      read    rio     rm
 and
 .I \*(yy
 are hard to type in shells other than
-.IR rc (1).
+.IM rc (1) .
 .\" and in troff!