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/pem.1

@@ -20,7 +20,7 @@ Privacy Enhanced Mail program but now commonly used for
 other applications, notably TLS.
 PEM encodes data in base 64
 (see
-.IR encode (3))
+.IM encode (3) )
 between lines of the form:
 .IP
 .EX
@@ -33,7 +33,7 @@ where
 may be any string describing the encoded data.
 The most common use of PEM format on Plan 9 is for encoding
 X.509 certificates; see
-.IR rsa (1).
+.IM rsa (1) .
 .PP
 .I Pemdecode
 extracts the named
@@ -62,4 +62,4 @@ hello world
 .SH SOURCE
 .B \*9/src/cmd/auth
 .SH "SEE ALSO
-.IR rsa (1)
+.IM rsa (1)