Fix Workhorse linter warnings (part 2)

Problem

make golangci linter detects warnings in following Workhorse files

How to verify

make golangci | grep -E "^cmd/"

cmd/gitlab-resize-image/main.go:1:1: package-comments: should have a package comment (revive)
cmd/gitlab-resize-image/png/reader.go:1:1: package-comments: should have a package comment (revive)
cmd/gitlab-resize-image/png/reader.go:26:1: exported: exported function NewReader should have comment or be unexported (revive)
cmd/gitlab-resize-image/png/reader.go:78:17: var-declaration: should omit type []byte from declaration of var magicBytes; it will be inferred from the right-hand side (revive)
cmd/gitlab-zip-cat/main.go:19:5: exported: exported var Version should have comment or be unexported (revive)
cmd/gitlab-zip-cat/main.go:1:1: package-comments: should have a package comment (revive)
cmd/gitlab-zip-cat/main.go:66:20: Error return value of `reader.Close` is not checked (errcheck)
cmd/gitlab-zip-cat/main.go:72:15: G110: Potential DoS vulnerability via decompression bomb (gosec)
cmd/gitlab-zip-cat/main.go:93:9: superfluous-else: if block ends with call to os.Exit function, so drop this else and outdent its block (revive)
cmd/gitlab-zip-metadata/limit/reader.go:1:1: package-comments: should have a package comment (revive)
cmd/gitlab-zip-metadata/limit/reader.go:22:1: exported: exported method LimitedReaderAt.ReadAt should have comment or be unexported (revive)
cmd/gitlab-zip-metadata/limit/reader.go:39:1: exported: exported function NewLimitedReaderAt should have comment or be unexported (revive)
cmd/gitlab-zip-metadata/limit/reader.go:9:5: exported: exported var ErrLimitExceeded should have comment or be unexported (revive)
cmd/gitlab-zip-metadata/main.go:16:5: exported: exported var Version should have comment or be unexported (revive)
cmd/gitlab-zip-metadata/main.go:1:1: package-comments: should have a package comment (revive)
cmd/gitlab-zip-metadata/main.go:64:9: superfluous-else: if block ends with call to os.Exit function, so drop this else and outdent its block (revive)

Solution

Resolve warnings