Fix checking C code formatting

Apparently, scripts/validate-c is not working in CI (or maybe
maintainers ignored the failures from it) -- current C code
gets some changes if we run indent on it.

This commit fixes this, simplifying things along the way.

 In particular:

1. Remove "validate" make target, add "cfmt" target that just runs
   indent on all *.c files in the repository (NOTE that *.h files
   are not included, as before).

   This may help a contributor to fix their code -- they just need
   to run "make cfmt" now instead of running "make validate" and
   copy-pasting the indent command and options from the hint.

2. Split GHA validate/misc into validate/release and validate/cfmt.
   The latter checks that the sources are not changed after "make cfmt".

3. Adds a few more options to indent. This was mostly motivated by
   trying to save the existing formatting, minimizing the amount of
   changes indent produces.

   The new options are:

   * -il0: sets the offset for goto labels to 0 (currently all labels
     but one are not indented -- let's keep it that way);

   * -ppi2: sets the indentation for nested preprocessor directives
     to 2 spaces (same as it is done in "SYS_memfd_create" defines);

   * -cp1: sets the indentation between #else / #endif and the
     following comment to 1 space.

4. Reformat the code using the new indent options.

5. Remove the now-unused script/{.validate,validate-c}.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/validate.yml

@@ -126,7 +126,7 @@ jobs:
       run: make cross
 
 
-  misc:
+  cfmt:
     runs-on: ubuntu-20.04
     steps:
     - name: checkout
@@ -136,8 +136,23 @@ jobs:
     - name: install deps
       run: |
         sudo apt -qq update
-        sudo apt -qq install libseccomp-dev indent
-    - name: make validate
-      run: make validate
+        sudo apt -qq install indent
+    - name: cfmt
+      run: |
+        make cfmt
+        git diff --exit-code
+
+
+  release:
+    runs-on: ubuntu-20.04
+    steps:
+    - name: checkout
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - name: install deps
+      run: |
+        sudo apt -qq update
+        sudo apt -qq install libseccomp-dev
     - name: make release
       run: make release