IOS 开发过程中常用的系统 函数

获取系统文件路径：

FOUNDATION_EXPORT NSString *NSUserName(void);

FOUNDATION_EXPORT NSString *NSFullUserName(void);

FOUNDATION_EXPORT NSString *NSHomeDirectory(void);

FOUNDATION_EXPORT NSString * __nullable NSHomeDirectoryForUser(NSString * __nullable userName);

FOUNDATION_EXPORT NSString *NSTemporaryDirectory(void);

FOUNDATION_EXPORT NSString *NSOpenStepRootDirectory(void);