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.
Also, if CC9FLAGS includes -fsanitize=address (ASAN),
predefine PLAN9PORT_ASAN for use by programs that need
to know (mainly libthread).
The 9c script used to have a variable called ngflags, which
was ccflags except -g (ng stood for "no g"), but nothing needs
it split out anymore, so simplify to just ccflags.
