enable all gocritic options

[thaJeztah]

• stacked on mount: fix ensureUnmount gated by build-tag #253
• stacked on chore: enable and fixes rules from gocritic #246

These were the failures;

mountinfo/mountinfo_bsd.go:21:2: rangeValCopy: each iteration copies 2168 bytes (consider pointers or indexing) (gocritic)
	for _, entry := range entries {
	^

mount/flags_unix.go:118:1: unnamedResult: consider giving a name to these results (gocritic)
func parseOptions(options string) (int, string) {
^

signal/signal.go:44:2: importShadow: shadow of imported package 'signal' (gocritic)
	signal, ok := SignalMap[strings.TrimPrefix(strings.ToUpper(rawSignal), "SIG")]
	^

user/idtools.go:52:1: unnamedResult: consider giving a name to these results (gocritic)
func getRootUIDGID(uidMap, gidMap []IDMap) (int, int, error) {
^
user/idtools.go:106:1: unnamedResult: consider giving a name to these results (gocritic)
func (i IdentityMapping) RootPair() (int, int) {
^
user/idtools.go:113:1: unnamedResult: consider giving a name to these results (gocritic)
func (i IdentityMapping) ToHost(uid, gid int) (int, int, error) {
^
user/idtools_unix.go:59:6: sloppyReassign: re-assignment to `err` can be replaced with `err := os.MkdirAll(path, mode)` (gocritic)
		if err = os.MkdirAll(path, mode); err != nil {
		   ^
user/idtools_unix.go:62:12: sloppyReassign: re-assignment to `err` can be replaced with `err := os.Mkdir(path, mode)` (gocritic)
	} else if err = os.Mkdir(path, mode); err != nil {
	          ^
user/idtools_unix.go:68:6: sloppyReassign: re-assignment to `err` can be replaced with `err := setPermissions(pathComponent, mode, uid, gid, nil)` (gocritic)
		if err = setPermissions(pathComponent, mode, uid, gid, nil); err != nil {
		   ^
user/idtools_unix.go:124:39: hugeParam: usr is heavy (96 bytes); consider passing it by pointer (gocritic)
func lookupSubRangesFile(path string, usr User) ([]IDMap, error) {
                                      ^
user/user_test.go:142:12: appendAssign: append result not assigned to the same slice (gocritic)
			data := append(pad, beyond...)
			        ^
user/user_utils.go:90:28: hugeParam: u is heavy (96 bytes); consider passing it by pointer (gocritic)
func (ua *userArg) matches(u User) bool {
                           ^

[thaJeztah]

Ah, it found some more; wondering if it can be configured to not require an explanation for errcheck 🤔

Error: mount/mount_unix_test.go:211:3: whyNoLint: include an explanation for nolint directive (gocritic)
		//nolint:errcheck
		^
Error: mount/mount_unix_test.go:212:3: deferInLoop: Possible resource leak, 'defer' is called in the 'for' loop (gocritic)
		defer Unmount(dir)
		^
Error: mount/mount_unix_test.go:262:3: deferInLoop: Possible resource leak, 'defer' is called in the 'for' loop (gocritic)
		defer Unmount(dir) //nolint:errcheck
		^
Error: mount/mounter_linux_test.go:109:1: paramTypeCombine: func(t *testing.T, mnt string, opts, optional, vfs string) could be replaced with func(t *testing.T, mnt, opts, optional, vfs string) (gocritic)