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/genrandom.3

@@ -25,11 +25,11 @@ number generator.  The X9.17 generator is seeded by 24
 truly random bytes read via
 .I truerand
 (see
-.IR rand (3)).
+.IM rand (3) ).
 .PP
 .I Prng
 uses the native
-.IR rand (3)
+.IM rand (3)
 pseudo-random number generator to fill the buffer.  Used with
 .IR srand ,
 this function can produce a reproducible stream of pseudo random
@@ -38,8 +38,8 @@ numbers useful in testing.
 Both functions may be passed to
 .I mprand
 (see
-.IR mp (3)).
+.IM mp (3) ).
 .SH SOURCE
 .B \*9/src/libsec
 .SH SEE ALSO
-.IR mp (3)
+.IM mp (3)