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/getuser.3

@@ -19,7 +19,7 @@ name of the user who
 owns the current process.
 .I Getuser
 calls
-.IR getuid (2)
+.IM getuid (2)
 and then reads
 .B /etc/passwd
 to find the corresponding name.
@@ -33,7 +33,7 @@ looks first for an environment variable
 If there is no such variable,
 .I sysname
 calls
-.IR gethostname (2)
+.IM gethostname (2)
 and truncates the returned name at the first dot.
 If
 .I gethostname