criu: introduce clang-format to format source code

This is another attempt to introduce a tool to format CRIU's source
code. This time it is based on clang-format.

The .clang-format file is taken from the linux kernel git tree (5.13).

I removed all comments from lines which state that it requires at least
clang-format 4 or 5. For this resulting file at least clang-format 11
is required. See scripts/fetch-clang-format.sh for all the changes
done to the Linux kernel .clang-format file.

Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Adrian Reber <areber@redhat.com>

Makefile

@@ -202,7 +202,7 @@ criu-deps	+= include/common/asm
 #
 # Configure variables.
 export CONFIG_HEADER := include/common/config.h
-ifeq ($(filter tags etags cscope clean mrproper,$(MAKECMDGOALS)),)
+ifeq ($(filter tags etags cscope clean lint indent fetch-clang-format help mrproper,$(MAKECMDGOALS)),)
 include Makefile.config
 else
 # To clean all files, enable make/build options here
@@ -398,6 +398,8 @@ help:
 	@echo '      test            - Run zdtm test-suite'
 	@echo '      gcov            - Make code coverage report'
 	@echo '      unittest        - Run unit tests'
+	@echo '      lint            - Run code linters'
+	@echo '      indent          - Indent C code'
 .PHONY: help
 
 lint:
@@ -429,6 +431,14 @@ codecov:
 	bash <(curl -s https://codecov.io/bash)
 .PHONY: codecov
 
+fetch-clang-format: .FORCE
+	$(E) ".clang-format"
+	$(Q) scripts/fetch-clang-format.sh
+
+indent:
+	find . -name '*.[ch]' -type f -print0 | xargs --null --max-args 128 --max-procs 4 clang-format -i
+.PHONY: indent
+
 include Makefile.install
 
 .DEFAULT_GOAL := all