对acl_vstring_mmap_alloc2和mmap_buf_init的改进与增加只读映射功能

主要改进根据offset调整到只分配所可操作大小的内存，增加只读文件的映射。发现一个很矛盾的问题，string创建时就加了一个'\0'结尾，一但写方式映射文件后要多为'\0'分配一个字节文件空间。如果文件是以二进制操作，这个'\0'就没意义了，[],copy方法调用后都是多了个'\0'，还会写到文件，最后只能用memcpy,memset。
```
static void mmap_buf_init(ACL_VSTRING *vp, size_t offset,int enablewrite)
{
#ifdef ACL_UNIX
	/*
	if (acl_lseek(vp->vbuf.fd, vp->vbuf.len, SEEK_SET) != vp->vbuf.len) {
		acl_msg_fatal("lseek failed: %s, off: %ld",
			acl_last_serror(), (long) vp->vbuf.len);
	}
	if (acl_file_write(vp->vbuf.fd, "\0", 1, 0, NULL, NULL) == ACL_VSTREAM_EOF) {
		acl_msg_fatal("write error: %s", acl_last_serror());
	}
	*/
#endif

#ifdef ACL_UNIX
	int swrite = enablewrite ? PROT_WRITE : 0;
	vp->vbuf.data = (unsigned char*) mmap(NULL, vp->maxlen,
		PROT_READ | swrite, MAP_SHARED, vp->vbuf.fd, offset);
	if ((void *) vp->vbuf.data == MAP_FAILED) {
		acl_msg_fatal("mmap error: %s", acl_last_serror());
	}
#elif defined(_WIN32) || defined(_WIN64)
	vp->vbuf.hmap = CreateFileMapping(vp->vbuf.fd, NULL, PAGE_READWRITE, 0,
		(DWORD) vp->maxlen, NULL);
	if (vp->vbuf.hmap == NULL) {
		acl_msg_fatal("CreateFileMapping: %s", acl_last_serror());
	}
	int swrite = enablewrite ? FILE_MAP_WRITE : 0;
	DWORD dwFileOffsetHigh = offset / (unsigned int) -1;
	DWORD dwFileOffsetLow  = offset % (unsigned int) -1;
	vp->vbuf.data = (unsigned char *) MapViewOfFile(vp->vbuf.hmap,
		FILE_MAP_READ | swrite, dwFileOffsetHigh,
		dwFileOffsetLow, 0);
	if (vp->vbuf.data == NULL) {
		acl_msg_fatal("MapViewOfFile error: %s", acl_last_serror());
	}
#else
	acl_msg_fatal("%s: not supported yet!", __FUNCTION__);
#endif
}
```

```
ACL_VSTRING *acl_vstring_mmap_alloc2(ACL_FILE_HANDLE fd,
	size_t max_len, size_t init_len, size_t offset, int enablewrite)
{
	const char *myname = "acl_vstring_mmap_alloc";
	ACL_VSTRING *vp;

if (init_len < 1) {
		acl_msg_panic("%s: bad length %ld", myname, (long) init_len);
	}

if (offset > 0) {
		acl_pthread_once(&__once_control, get_pagesize);
		int n = offset % __page_size  == 0 ? 0 : 1;
		offset = (n + offset / __page_size) * __page_size;
	}

if (max_len < init_len) {
		max_len = init_len;
	}

if (max_len < offset) {
		max_len = offset;
	}

vp = (ACL_VSTRING *) acl_mymalloc(sizeof(*vp));

vp->vbuf.fd = fd;
	vp->slice   = NULL;
	vp->dbuf    = NULL;

vp->vbuf.flags     = 0;
	vp->vbuf.len       = init_len - offset + 1;//-offset:减少分配操作不到的0到offset内存,+1:方便在未位写'\0'
	vp->vbuf.get_ready = vstring_buf_get_ready;
	vp->vbuf.put_ready = vstring_buf_put_ready;
	vp->vbuf.space = vstring_buf_space;
/*	vp->vbuf.ctx = NULL; */
	vp->maxlen         = max_len - offset + 1;

mmap_buf_init(vp, offset, enablewrite);

ACL_VSTRING_RESET(vp);
	//vp->vbuf.data[0] = 0; //在映射文件后这里写入一个'\0'到文件，破坏了文件内容
	return vp;
}
```

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