MDEV-21192: SST failing when enabling IPV6

The following features have been added:

1) Automatic addition of the pf = ip6 option for socat
   when it can be recognized by the format of the connection
   address;
2) Automatically add or remove extra commas at the beginning
   and at the end of sockopt, for example, sockopt='pf=ip6'
   and sockopt=',pf=ip6' work equally well;

Also, due to interference in the code of the get_transfer()
function, I also refactored it and now:

3) encrypt = 4 is supported not only for xtrabackup-v2,
   but also for mariabackup - this can help with migration
   from Percona;
4) Improved setting of 'commonname' option for encrypt=3
   and encrypt=4 modes;

mysql-test/suite/galera_3nodes/t/galera_ipv6_mariabackup.cnf

@@ -34,4 +34,3 @@ bind-address=::
 [SST]
 transferfmt=@ENV.MTR_GALERA_TFMT
 streamfmt=xbstream
-sockopt=",pf=ip6"

mysql-test/suite/galera_3nodes/t/galera_ipv6_mariabackup_section.cnf

@@ -38,4 +38,5 @@ bind-address=::
 [SST]
 transferfmt=@ENV.MTR_GALERA_TFMT
 streamfmt=xbstream
+# Not needed, but left here for debugging:
 sockopt=",pf=ip6"

mysql-test/suite/galera_3nodes/t/galera_ipv6_mysqldump.cnf

@@ -24,6 +24,3 @@ wsrep_provider_options='base_host=[::1];base_port=@mysqld.3.#galera_port;gmcast.
 wsrep_sst_receive_address='[::1]:@mysqld.3.#sst_port'
 wsrep_node_incoming_address='[::1]:@mysqld.3.port'
 bind-address=::
-
-[SST]
-sockopt=",pf=ip6"

mysql-test/suite/galera_3nodes/t/galera_ipv6_rsync.cnf

@@ -24,6 +24,3 @@ wsrep_provider_options='base_host=[::1];base_port=@mysqld.3.#galera_port;gmcast.
 wsrep_sst_receive_address='[::1]:@mysqld.3.#sst_port'
 wsrep_node_incoming_address='[::1]:@mysqld.3.port'
 bind-address=::
-
-[SST]
-sockopt=",pf=ip6"

mysql-test/suite/galera_3nodes/t/galera_ipv6_rsync_section.cnf

@@ -29,6 +29,3 @@ wsrep_provider_options='base_host=[::1];base_port=@mysqld.3.#galera_port;gmcast.
 wsrep_sst_receive_address='[::1]:@mysqld.3.#sst_port'
 wsrep_node_incoming_address='[::1]:@mysqld.3.port'
 bind-address=::
-
-[SST]
-sockopt=",pf=ip6"

scripts/wsrep_sst_common.sh

@@ -1021,3 +1021,25 @@ check_for_version()
     [ $z1 -lt $z2 ] && return 1
     return 0
 }
+
+trim_string()
+{
+    if [ -n "$BASH_VERSION" ]; then
+        local pattern="[![:space:]${2:-}]"
+        local x="${1#*$pattern}"
+        local z=${#1}
+        x=${#x}
+        if [ $x -ne $z ]; then
+            local y="${1%$pattern*}"
+            y=${#y}
+            x=$(( $z-$x-1 ))
+            y=$(( $y-$x+1 ))
+            printf '%s' "${1:$x:$y}"
+        else
+            printf ''
+        fi
+    else
+        local pattern="[[:space:]${2:-}]"
+        echo "$1" | sed -E "s/^$pattern+|$pattern+\$//g"
+    fi
+}

scripts/wsrep_sst_rsync.sh

@@ -261,16 +261,20 @@ then
         ;;
     'VERIFY_CA')
         VERIFY_OPT='verifyChain = yes'
-        # check if the address is an ip-address (v4 or v6):
-        if echo "$WSREP_SST_OPT_HOST_UNESCAPED" | \
-           grep -q -E '^([0-9]+(\.[0-9]+){3,3}|[0-9a-fA-F]?(\:[0-9a-fA-F]*)+)$'
-        then
-            CHECK_OPT="checkIP = $WSREP_SST_OPT_HOST_UNESCAPED"
+        if [ -n "$WSREP_SST_OPT_REMOTE_USER" ]; then
+            CHECK_OPT="checkHost = $WSREP_SST_OPT_REMOTE_USER"
         else
-            CHECK_OPT="checkHost = $WSREP_SST_OPT_HOST"
-        fi
-        if is_local_ip "$WSREP_SST_OPT_HOST_UNESCAPED"; then
-            CHECK_OPT_LOCAL="checkHost = localhost"
+            # check if the address is an ip-address (v4 or v6):
+            if echo "$WSREP_SST_OPT_HOST_UNESCAPED" | \
+               grep -q -E '^([0-9]+(\.[0-9]+){3,3}|[0-9a-fA-F]*(\:[0-9a-fA-F]*)+)$'
+            then
+                CHECK_OPT="checkIP = $WSREP_SST_OPT_HOST_UNESCAPED"
+            else
+                CHECK_OPT="checkHost = $WSREP_SST_OPT_HOST"
+            fi
+            if is_local_ip "$WSREP_SST_OPT_HOST_UNESCAPED"; then
+                CHECK_OPT_LOCAL="checkHost = localhost"
+            fi
         fi
         ;;
     *)