Optimize Net module for Android

[zhuzeitou]

1. Poco uses epoll_create, epoll_ctl and epoll_wait, and they are always available for NDK. So we can enable epoll for Android platform.

int epoll_create(int __size);

#if __ANDROID_API__ >= 21
int epoll_create1(int __flags) __INTRODUCED_IN(21);
#endif /* __ANDROID_API__ >= 21 */


int epoll_ctl(int __epoll_fd, int __op, int __fd, struct epoll_event* __BIONIC_COMPLICATED_NULLNESS __event);
int epoll_wait(int __epoll_fd, struct epoll_event* _Nonnull __events, int __event_count, int __timeout_ms);

#if __ANDROID_API__ >= 21
int epoll_pwait(int __epoll_fd, struct epoll_event* _Nonnull __events, int __event_count, int __timeout_ms, const sigset_t* _Nullable __mask) __INTRODUCED_IN(21);
#endif /* __ANDROID_API__ >= 21 */


#if __ANDROID_API__ >= 28
int epoll_pwait64(int __epoll_fd, struct epoll_event* _Nonnull __events, int __event_count, int __timeout_ms, const sigset64_t* _Nullable __mask) __INTRODUCED_IN(28);
#endif /* __ANDROID_API__ >= 28 */

2. <ifaddr.h>(getifaddrs and freeifaddrs) was introduced in android-24, so we can use __ANDROID_API__ >= 24 to check if they are supported

#if __ANDROID_API__ >= 24
int getifaddrs(struct ifaddrs* _Nullable * _Nonnull __list_ptr) __INTRODUCED_IN(24);

/**
 * [freeifaddrs(3)](http://man7.org/linux/man-pages/man3/freeifaddrs.3.html) frees a linked list
 * of `struct ifaddrs` returned by getifaddrs().
 *
 * Available since API level 24.
 */
void freeifaddrs(struct ifaddrs* _Nullable __ptr) __INTRODUCED_IN(24);
#endif /* __ANDROID_API__ >= 24 */