Makefile, dot/rcrc: Hack rc(1) profile for Plan 9 compatibility.

Makefile, dot/rcrc: Hack rc(1) profile for Plan 9 compatibility.

Annoyingly, the native-Linux rc(1) port doesn't accept the same syntax
as Plan 9 From User Space.

  * Plan 9 requires braces around command substitutions, so `{hostname}
    rather than `hostname.

  * Plan 9 doesn't have here-strings, so use echo instead.

  * Plan 9 is pickier about concatenating unset variables, so set them
    empty explicitly.

  * Plan 9 has a bizarre `if not' syntax rather than allowing `else'
    clauses.

This last is the most annoying difference, since the only thing
acceptable to both implementations is to repeat the condition.

Prefer the Plan 9 version throughout, and seddery the script to make the
native port version.  Fortunately they end up in different places;
otherwise we'd be really stuffed.