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.
The code is adapted from Plan 9's import(4); this allows us to speak
that protocol. We don't currently support AAN (in the works) or
TLS/SSL.
Thanks to David for help with the man page, testing, and development.
R=0intro, rsc
CC=plan9port.codebot
http://codereview.appspot.com/6458100