Fix trivial style errors reported by go vet and golint

No substantial code change.
Note that some style errors reported by `golint` are not fixed due to possible compatibility issues.

Signed-off-by: Akihiro Suda <suda.kyoto@gmail.com>

libcontainer/stacktrace/capture.go

@@ -2,14 +2,14 @@ package stacktrace
 
 import "runtime"
 
-// Caputure captures a stacktrace for the current calling go program
+// Capture captures a stacktrace for the current calling go program
 //
 // skip is the number of frames to skip
 func Capture(userSkip int) Stacktrace {
 	var (
 		skip   = userSkip + 1 // add one for our own function
 		frames []Frame
-		prevPc uintptr = 0
+		prevPc uintptr
 	)
 	for i := skip; ; i++ {
 		pc, file, line, ok := runtime.Caller(i)