pycrate_osmo.RRCTL: reserve a range for RRCTL protocol extensions

The idea is to group protocol extensions and have one message
type (with 4 variations: Req/Ind/Cnf/Err) per group.  Similar
to the stack and heap allocation model in Linux, new entries
shall be added to the 'RRCTLMsgType' enum as follows:

  * regular messages - from the bottom downward,
  * extension groups - from the end upward.

The mask '11xxxx'B gives us 16 unique messages, one is reserved.

pycrate_osmo/RRCTL.py

@@ -51,6 +51,8 @@ class RRCTLMsgType(enum.IntEnum):
     Paging                      = 0x06
     Param                       = 0x07
     SecMode                     = 0x08
+    # RRCTL protocol extensions (0b11xxxx) follow
+    RFU                         = 0x3f # 0b111111
 
 RRCTLMsgType_dict = { e.value : e.name for e in RRCTLMsgType }