[sr-dev] [kamailio/kamailio] tm remap_503_500 breaks dialogs (#2405)

[geejay101]

### Description

Apparently the remapping of 503 to 500 codes in the tm module does also change the to-tag. This behaviour breaks dialogs with yate and therefore calls hang and the 503 remains unacknowledged. 

Removing to-tags seems to contradict RFC3261

"12 Dialogs
A dialog is identified at each UA with a dialog ID, which consists of a Call-ID value, a local tag **and a remote tag**…"

Due to the bug the UA cannot identify the dialog the 500 belongs to. Therefore the dialog remains open and Kamailio keeps repeating 500 messages.

### Troubleshooting
After disabling the 503 to 500 remapping with modparam("tm", "remap_503_500", 0) dialogs get properly terminated .

#### Reproduction

503 to 500 remapping is enabled by default. Inspect the to tags of the incoming 503 and outgoing 500 messages.

#### Debugging Data

Henning added this comment:

Apparently, this is the way the code works:
 
t_reply.c:
                        if (relayed_code==503 && tm_remap_503_500){
                                /* replace a final 503 with a 500:
                                 * generate a "FAKE" reply and a new to_tag (for easier
                                 *  debugging)*/
 
### Possible Solutions

Dont change the to-tag

### Additional Information

kamailio -v
version: kamailio 5.2.3 (x86_64/linux) c36229
flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES
ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144 MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: c36229 
compiled on 10:34:54 Jun 13 2019 with gcc 4.8.5

* **Operating System**:

CentOS Linux release 7.6.1810 (Core) 
3.10.0-957.12.2.el7.x86_64


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