Migrate extensions: *.r => *.reb & *.rx => *.rebx

Tested on OS X 10.8.4 like this:
make clean
make prep
make make
make
./r3   # it has booted up successfully

---==| ~/p/rebol-test |==---
$ ~/p/rebol/make/r3 run-recover.r
Testing ...
file: %core-tests.r
recovering at: 4687
Done, see the log file: /Users/onetom/p/rebol-test/r_2_101_0_4_4_C8BB7C_4E5726.log

system/version: 2.101.0.4.4
code-checksum: #{C8BB7C9E4EFD268B5B378F76C67C3DE61969DDD0}
test-checksum: #{4E5726B3CE4645AC5EAB8AF08F677090E0998DD3}
Total: 4696
Succeeded: 4213
Test-failures: 159
Crashes: 16
Dialect-failures: 0
Skipped: 308