shared/format-table: use enum instead of Table.empty_string

All users were setting this to some static string (usually "-"), so let's
simplify things by not doing strdup, but instead limiting callers to a fixed
set of values. In preparation for the next commit, the function is renamed from
"empty" to "replacement", because it'll be used for more than empty fields. I
didn't do the whole string-table setup, because it's all used internally in one
file and this way we can immediately assert if an invalid value is passed in.

Some callers were (void)ing the error, others were ignoring it, and others
propagating. It's nicer to remove the boilerplate.

src/shared/format-table.c

@@ -133,6 +133,8 @@ struct Table {
         size_t n_cells;
 
         bool header;   /* Whether to show the header row? */
+        TableErsatz ersatz; /* What to show when we have an empty cell or an invalid value that cannot be rendered. */
+
         size_t width;  /* If == 0 format this as wide as necessary. If SIZE_MAX format this to console
                         * width or less wide, but not wider. Otherwise the width to format this table in. */
         size_t cell_height_max; /* Maximum number of lines per cell. (If there are more, ellipsis is shown. If SIZE_MAX then no limit is set, the default. == 0 is not allowed.) */
@@ -149,8 +151,6 @@ struct Table {
         size_t n_json_fields;
 
         bool *reverse_map;
-
-        char *empty_string;
 };
 
 Table *table_new_raw(size_t n_columns) {
@@ -167,6 +167,7 @@ Table *table_new_raw(size_t n_columns) {
                 .header = true,
                 .width = SIZE_MAX,
                 .cell_height_max = SIZE_MAX,
+                .ersatz = TABLE_ERSATZ_EMPTY,
         };
 
         return TAKE_PTR(t);
@@ -242,7 +243,6 @@ Table *table_unref(Table *t) {
         free(t->display_map);
         free(t->sort_map);
         free(t->reverse_map);
-        free(t->empty_string);
 
         for (size_t i = 0; i < t->n_json_fields; i++)
                 free(t->json_fields[i]);
@@ -1089,10 +1089,26 @@ void table_set_cell_height_max(Table *t, size_t height) {
         t->cell_height_max = height;
 }
 
-int table_set_empty_string(Table *t, const char *empty) {
+void table_set_ersatz_string(Table *t, TableErsatz ersatz) {
         assert(t);
+        assert(ersatz >= 0 && ersatz < _TABLE_ERSATZ_MAX);
 
-        return free_and_strdup(&t->empty_string, empty);
+        t->ersatz = ersatz;
+}
+
+static const char* table_ersatz_string(const Table *t) {
+        switch (t->ersatz) {
+        case TABLE_ERSATZ_EMPTY:
+                return "";
+        case TABLE_ERSATZ_DASH:
+                return "-";
+        case TABLE_ERSATZ_UNSET:
+                return "(unset)";
+        case TABLE_ERSATZ_NA:
+                return "n/a";
+        default:
+                assert_not_reached();
+        }
 }
 
 static int table_set_display_all(Table *t) {
@@ -1397,7 +1413,7 @@ static const char *table_data_format(Table *t, TableData *d, bool avoid_uppercas
 
         switch (d->type) {
         case TABLE_EMPTY:
-                return strempty(t->empty_string);
+                return table_ersatz_string(t);
 
         case TABLE_STRING:
         case TABLE_PATH:
@@ -1418,7 +1434,7 @@ static const char *table_data_format(Table *t, TableData *d, bool avoid_uppercas
 
         case TABLE_STRV:
                 if (strv_isempty(d->strv))
-                        return strempty(t->empty_string);
+                        return table_ersatz_string(t);
 
                 d->formatted = strv_join(d->strv, "\n");
                 if (!d->formatted)
@@ -1427,7 +1443,7 @@ static const char *table_data_format(Table *t, TableData *d, bool avoid_uppercas
 
         case TABLE_STRV_WRAPPED: {
                 if (strv_isempty(d->strv))
-                        return strempty(t->empty_string);
+                        return table_ersatz_string(t);
 
                 char *buf = format_strv_width(d->strv, column_width);
                 if (!buf)