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/man4/fontsrv.4

@@ -20,7 +20,7 @@ fontsrv \- file system access to host fonts
 presents the host window system's fonts
 in the standard Plan 9 format
 (see
-.IR font (7)).
+.IM font (7) ).
 It serves a virtual directory tree mounted at
 .I mtpt
 (if the
@@ -72,14 +72,14 @@ representing 32-character Unicode ranges.
 .PP
 .I Openfont
 (see
-.IR graphics (3))
+.IM graphics (3) )
 recognizes font paths beginning with
 .B /mnt/font
 and implements them by invoking
 .IR fontsrv ;
 it need not be running already.
 See
-.IR font (7)
+.IM font (7)
 for a full discussion of font name syntaxes.
 .SH EXAMPLES
 List the fonts on the system:
@@ -96,7 +96,7 @@ or:
 .EE
 .LP
 Run
-.IR acme (1)
+.IM acme (1)
 using the operating system's Monaco as the fixed-width font:
 .IP
 .EX
@@ -104,7 +104,7 @@ using the operating system's Monaco as the fixed-width font:
 .EE
 .LP
 Run
-.IR sam (1)
+.IM sam (1)
 using the same font:
 .IP
 .EX
@@ -113,7 +113,7 @@ using the same font:
 .SH SOURCE
 .B \*9/src/cmd/fontsrv
 .SH SEE ALSO
-.IR font (7)
+.IM font (7)
 .SH BUGS
 .PP
 Due to OS X restrictions,