Fix problems signaled by new linter version v1.45.2

PR (linter: upgrade linter version #405) triggered lint job which failed
with many errors generated by various linters. Configurations were added to
golangcilint.yaml and several refactorings were made in order to improve the
results of the linter.

maintidx linter disabled

Signed-off-by: Alex Stan <alexandrustan96@yahoo.ro>

Makefile

@@ -9,6 +9,7 @@ TOOLSDIR := $(shell pwd)/hack/tools
 PATH := bin:$(TOOLSDIR)/bin:$(PATH)
 STACKER := $(shell which stacker)
 GOLINTER := $(TOOLSDIR)/bin/golangci-lint
+GOLINTER_VERSION := v1.45.2
 NOTATION := $(TOOLSDIR)/bin/notation
 BATS := $(TOOLSDIR)/bin/bats
 TESTDATA := $(TOP_LEVEL)/test/data
@@ -89,7 +90,7 @@ covhtml:
 
 $(GOLINTER):
 	mkdir -p $(TOOLSDIR)/bin
-	curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(TOOLSDIR)/bin v1.43.0
+	curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(TOOLSDIR)/bin $(GOLINTER_VERSION)
 	$(GOLINTER) version
 
 .PHONY: check