Reserve {{W_RV}} and {{W_SR}} FITS card prefixes

PFS instrument cards are required to be 8 characters, beginning with "W_". So far (for MEB, MCP, XCU, ENU, DCB) the subsystem takes one or two characters, then smashes all the meaningful parts into an incomprehensible clot.

I'd like to reserve a couple of prefixes for cross-actor names, specifically for (git) versions and serial numbers: W_RV and W_SR, with REVISION_ and SERIAL_ for the long name prefixes.