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/bio.3

@@ -93,7 +93,7 @@ for mode
 or creates for mode
 .BR OWRITE .
 It calls
-.IR malloc (3)
+.IM malloc (3)
 to allocate a buffer.
 .PP
 .I Bfdopen
@@ -104,7 +104,7 @@ for mode
 or
 .BR OWRITE .
 It calls
-.IR malloc (3)
+.IM malloc (3)
 to allocate a buffer.
 .PP
 .I Binit
@@ -173,7 +173,7 @@ of the most recent string returned by
 .PP
 .I Brdstr
 returns a
-.IR malloc (3)-allocated
+.IM malloc (3) -allocated
 buffer containing the next line of input delimited by
 .IR delim ,
 terminated by a NUL (0) byte.
@@ -225,7 +225,7 @@ may back up a maximum of five bytes.
 uses
 .I charstod
 (see
-.IR atof (3))
+.IM atof (3) )
 and
 .I Bgetc
 to read the formatted
@@ -246,7 +246,7 @@ and a negative value is returned if a read error occurred.
 .PP
 .I Bseek
 applies
-.IR seek (3)
+.IM seek (3)
 to
 .IR bp .
 It returns the new file offset.
@@ -278,7 +278,7 @@ on the output stream.
 .PP
 .I Bprint
 is a buffered interface to
-.IR print (3).
+.IM print (3) .
 If this causes a
 .IR write
 to occur and there is an error,
@@ -325,10 +325,10 @@ written.
 .SH SOURCE
 .B \*9/src/libbio
 .SH SEE ALSO
-.IR open (3),
-.IR print (3),
-.IR exits (3),
-.IR utf (7),
+.IM open (3) ,
+.IM print (3) ,
+.IM exits (3) ,
+.IM utf (7) ,
 .SH DIAGNOSTICS
 .I Bio
 routines that return integers yield