This tells bin/man to set the register L to very high to avoid pagination
and updates tmac/tmac.an to use that value, if it's set, to set the page
length. This is per Plan 9's rc/bin/man and sys/lib/tmac/tmac.an.
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.
