tmac: rename IM (italic manual) to MR (manual reference) Suggested by G. Brandon Robinson.

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.

src/cmd: Add a repurposed import(4), called `9import', to the ports. 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