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/secstore.1

@@ -97,7 +97,7 @@ Option
 .B -n
 says that the password should be read from NVRAM
 (see
-.IR authsrv (3))
+.IM authsrv (3) )
 instead of from
 .BR /dev/tty .
 .PP
@@ -107,7 +107,7 @@ or the server specified by option
 .BR -s .
 .PP
 For example, to add a secret to the file read by
-.IR factotum (4),
+.IM factotum (4) ,
 run
 .sp
 .EX
@@ -192,8 +192,8 @@ block chaining (CBC) mode.
 .br
 .B \*9/src/cmd/auth/secstore
 .SH SEE ALSO
-.IR factotum (4),
-.IR secstored (1)
+.IM factotum (4) ,
+.IM secstored (1)
 .SH BUGS
 There is deliberately no backup of files on the secstore, so
 .B -r