Fix #60: `C::%UNARY-FROUND` is undefined

Remove the symbol `C::%UNARY-FROUND` and just let the compiler package
inherit it from the kernel package.

* src/bootfiles/21c/boot-2018-02-1.lisp
    * Use this to bootstrap the change (by uninterning the symbol)
* src/code/float.lisp
    * Just export %unary-fround
* src/general-info/release-21d.md
    * Update
* tests/issues.lisp
    * Add test for this issue
2 jobs for rtoy-fix-60-unary-fround-undefined in 4 minutes and 20 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #322
linux
linux-runner

00:04:20

failed #323
osx
osx-runner

00:03:13

 
Name Stage Failure
failed
linux-runner Test There is an unknown failure, please try again
                          ^
../../src/lisp/lisp.c:466:17: warning: variable 'unidata' set but not used [-Wunused-but-set-variable]
const char *unidata = NULL;
^
make: *** [lisp.o] Error 1
<builtin>: recipe for target 'lisp.o' failed
make: Leaving directory '/builds/cmucl/cmucl/linux-2/lisp'
Failed: make -C linux-2/lisp
ERROR: Job failed: exit code 1
failed
osx-runner Test There is an unknown failure, please try again
    ^
../../src/lisp/lisp.h:50:31: note: expanded from macro 'SetSymbolValue'
(((struct symbol *)((sym)-type_OtherPointer))->value = (val))
^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.
make: *** [lisp.o] Error 1
Failed: make -C darwin-2/lisp
ERROR: Job failed: exit status 1