fix!: reverse proxy to sub-directory (#3483)

from this commit, if you want reverse proxy to sub-directory like `alist` with `nginx`, you need config:

```nginx
location /alist/ {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header Host $http_host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header Range $http_range;
    proxy_set_header If-Range $http_if_range;
    proxy_redirect off;
    proxy_pass http://127.0.0.1:5244/alist/;
    # the max size of file to upload
    client_max_body_size 20000m;
}
```

internal/conf/var.go

@@ -1,6 +1,9 @@
 package conf
 
-import "regexp"
+import (
+	"net/url"
+	"regexp"
+)
 
 var (
 	BuiltAt    string
@@ -13,6 +16,7 @@ var (
 
 var (
 	Conf *Config
+	URL  *url.URL
 )
 
 var SlicesMap = make(map[string][]string)