Makefile: make "lint" target also lint cmd/continuity module and fix linting issues

[thaJeztah]

cmd/continuity: fix SA1019: entry.User/entry.Group is deprecated

Add nolint-comments, and a small utility to show either the user/group
name (if present), otherwise fall back to use Uid/Gid.

Before this patch:

cd cmd/continuity
go install
continuity build ./continuityfs/ > manifest.pb
continuity ls ./manifest.pb
-rw-r--r--      7.5 kB  /fuse.go
-rw-r--r--      1.7 kB  /provider.go

With this patch:

cd cmd/continuity
go install
continuity build ./continuityfs/ > manifest.pb
continuity ls ./manifest.pb
-rw-r--r--  501  20  7.5 kB  /fuse.go
-rw-r--r--  501  20  1.7 kB  /provider.go

cmd/continuity/continuityfs: SA1019: fuse.ENOENT is deprecated

Error: continuityfs/fuse.go:200:15: SA1019: fuse.ENOENT is deprecated: Return a syscall.Errno directly. See ToErrno for exact rules. (staticcheck)
        return nil, fuse.ENOENT
                    ^

Makefile: make "lint" target also lint cmd/continuity module

cmd/continuity is a separate module, and therefore was not linted.

[thaJeztah]

Ah! Two / Three remaining; I thought I had patches for that, but probably stashed them;

Error: continuityfs/fuse.go:200:15: SA1019: fuse.ENOENT is deprecated: Return a syscall.Errno directly. See ToErrno for exact rules. (staticcheck)
		return nil, fuse.ENOENT
		            ^
Error: commands/ls.go:48:76: SA1019: entry.User is deprecated: Do not use. (staticcheck)
					fmt.Fprintf(w, "%v\t%v\t%v\t%v\t%v -> %v\n", os.FileMode(entry.Mode), entry.User, entry.Group, humanize.Bytes(uint64(entry.Size)), path, entry.Target)
					                                                                      ^
Error: commands/ls.go:51:70: SA1019: entry.User is deprecated: Do not use. (staticcheck)
					fmt.Fprintf(w, "%v\t%v\t%v\t%v\t%v\n", os.FileMode(entry.Mode), entry.User, entry.Group, humanize.Bytes(uint64(entry.Size)), path)