Also update install.txt, which mistakenly contained intro(1). Pointed out by Nicholas Schwartz.

Suggested by G. Brandon Robinson.

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.

Linux 2.4 is dead. (The libthread code hasn't worked for Linux 2.4 for a long time.)

Change-Id: I940a21d67b21b5dd139cb20b9fae9595506fc5ae

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