fix: address ShellCheck findings SC2163 and SC2181

dyntls.sh

@@ -520,9 +520,8 @@ set_var() {
     shift
     value=$*
 
-    eval "${var}=\${$var:-$value}"
-
-    export "$var"
+    eval "$var=\${$var-\$value}"
+    export "${var?}"
 } #=> set_var()
 
 
@@ -548,7 +547,7 @@ set_list() {
         eval "$list=\"$value\""
     fi
 
-    export "$list"
+    export "${list?}"
 } #=> set_list()
 
 
@@ -1396,11 +1395,9 @@ _check_cert() {
     _log "Enter the function '_check_cert()'..." 1
 
     # Verify FQHN
-    _Hostname "check" "hostname"
-
-    if [ "$?" -ne 0 ]; then
-        _log "The provided hostname '$DYNTLS_MEMBER_HOSTNAME' is not a valid FQHN. Aborting check." 3
-        _log "Leave the function '_check_cert()'..." 1
+    if ! Hostname check hostname; then
+        log "The provided hostname $DYNTLS_MEMBER_HOSTNAME is not a valid FQHN. Aborting check." 3
+        log "Leave the function checkcert..." 1
         exit 1
     fi
 
@@ -1437,9 +1434,7 @@ _remove_cert() {
     _log "Enter the function '_remove_cert()'..." 1
 
     # Verify FQHN
-    _Hostname "check" "hostname"
-
-    if [ "$?" -ne 0 ]; then
+    if ! Hostname check hostname; then
         _log "The provided hostname '$DYNTLS_MEMBER_HOSTNAME' is not a valid FQHN. Cannot proceed." 3
         _log "Leave the function '_remove_cert()'..." 1
 
@@ -1447,8 +1442,7 @@ _remove_cert() {
     fi
 
     # Check HostnameDB
-    _HostnameDB "check"
-    if [ "$?" -eq 0 ]; then
+    if HostnameDB check; then
         # Remove hostname
         _log "Hostname '$DYNTLS_MEMBER_HOSTNAME' found. Removing from HostnameDB..." 1