bring back std/ prefix within compiler and ensure it works in bootstrap + bsd (#17902)

* [WIP] bring back std/ prefix within compiler and ensure it works in bootstrap + bsd

* refs https://github.com/nim-lang/Nim/pull/16282#discussion_r616846863
* sounds very similar to https://github.com/nim-lang/Nim/pull/14291
* more: vmops
* update tools/ci_generate.nim
* auto-generate freebsd.yml as well, to avoid duplication with openbsd.yml
* cleanup
* undo temporary CI removal

.builds/openbsd_0.yml

@@ -1,4 +1,4 @@
-## do not edit directly; auto-generated by `nim r tools/ci_generate.nim`
+## DO NO EDIT DIRECTLY! auto-generated by `nim r tools/ci_generate.nim`
 
 image: openbsd/latest
 packages:
@@ -10,6 +10,8 @@ packages:
 - sfml
 - sdl2
 - libffi
+
+
 sources:
 - https://github.com/nim-lang/Nim
 environment:
@@ -20,9 +22,10 @@ tasks:
     set -e
     cd Nim
     . ci/funs.sh && nimBuildCsourcesIfNeeded
-    $nim_csources c koch
+    $nim_csources c --skipUserCfg --skipParentCfg koch
     echo 'export PATH=$HOME/Nim/bin:$PATH' >> $HOME/.buildenv
 - test: |
+    set -e
     cd Nim
     if ! ./koch runCI; then
       nim r tools/ci_testresults.nim