Support value-in-range with <number> keyword

This allows syntax checking to pass for fields like vxlan-ttl/vxlan-tos
which can be a number in a range OR a string value representing a special
meaning (0-255 or "auto", for instance).  Without this, you can only pass
a --syntax-check for such fields if your value is one of those literally
specified because, for instance, "64" is not "auto", "0", or "255":

invalid value "64": valid attribute values: ['0', '255']
info: exit status 1

Note that _applying_ such configuration still works, because netlink's
acceptance criteria are independent of ifupdown2's.

ifupdown2/addons/vxlan.py

@@ -97,12 +97,14 @@ class vxlan(Addon, moduleBase):
                 "help": "specifies the TTL value to use in outgoing packets "
                         "(range 0..255), 0=auto",
                 "default": "0",
-                "validvals": ["0", "255"],
+                "validrange": ["0", "255"],
+                "validvals": ["<number>", "auto"],
                 "example": ['vxlan-ttl 42'],
             },
             "vxlan-tos": {
                 "help": "specifies the ToS value (range 0..255), 1=inherit",
-                "validvals": ["inherit", "0", "255"],
+                "validrange": ["0", "255"],
+                "validvals": ["<number>", "inherit"],
                 "example": ['vxlan-tos 42'],
             },
             "vxlan-mcastgrp": {

ifupdown2/ifupdown/ifupdownmain.py

@@ -1224,7 +1224,9 @@ class ifupdownMain:
 
     def _keyword_number(self, value, validrange=None):
         try:
-            int(value)
+            int_value = int(value)
+            if validrange is not None:
+                return int(validrange[0]) <= int_value <= int(validrange[1])
             return True
         except Exception as e:
             self.logger.debug('keyword: number: %s' % str(e))