panel: implement string_file_ext() for extension column display

The extension panel field used string_file_name as its display callback,
showing the full filename instead of just the extension. There was
already a TODO comment acknowledging this (// TODO: string_file_ext).

Add string_file_ext() which:
- Returns only the part after the last dot for regular files
- Returns an empty string for directories and extensionless files

Also document the extension field in the man page and NEWS.

Signed-off-by: Ilia Karpov <iakarpov@edu.hse.ru>
Signed-off-by: ki <karpovilia@gmail.com>

Author: karpovilia

doc/NEWS

@@ -32,6 +32,7 @@ Starting with this release, we will be using language features that require a C9
 
 - Fixes
 
+    * Extension column in user-defined panel format now displays only the file extension instead of the full filename
     * Segfault if filter makes file panel empty (#4600)
     * Segfault in built-in help when going to the previous topic (#4627)
     * Incorrect handling of ext2 attributes of a directory (#4590)

doc/man/mc.1.in

@@ -798,6 +798,10 @@ specifier.  This are the available fields you may display:
 .B name
 displays the file name.
 .TP
+.B extension
+displays the file extension (the part after the last dot).
+For directories and files without an extension, this field is empty.
+.TP
 .B size
 displays the file size.
 .TP

src/filemanager/panel.c

@@ -159,6 +159,7 @@ static const char *string_file_group (const file_entry_t *fe, int len);
 static const char *string_marked (const file_entry_t *fe, int len);
 static const char *string_space (const file_entry_t *fe, int len);
 static const char *string_dot (const file_entry_t *fe, int len);
+static const char *string_file_ext (const file_entry_t *fe, int len);
 
 /*** file scope variables ************************************************************************/
 
@@ -178,9 +179,7 @@ static panel_field_t panel_fields[] = {
     { "extension", 12, TRUE, J_LEFT_FIT,
       // TRANSLATORS: one single character to represent 'extension' sort mode
       // TRANSLATORS: no need to translate 'sort', it's just a context prefix
-      N_ ("sort|e"), N_ ("E&xtension"), TRUE, FALSE,
-      string_file_name,  // TODO: string_file_ext
-      (GCompareFunc) sort_ext },
+      N_ ("sort|e"), N_ ("E&xtension"), TRUE, FALSE, string_file_ext, (GCompareFunc) sort_ext },
     { "size", 7, FALSE, J_RIGHT,
       // TRANSLATORS: one single character to represent 'size' sort mode
       // TRANSLATORS: no need to translate 'sort', it's just a context prefix
@@ -355,6 +354,25 @@ string_file_name (const file_entry_t *fe, int len)
 
 /* --------------------------------------------------------------------------------------------- */
 
+static const char *
+string_file_ext (const file_entry_t *fe, int len)
+{
+    const char *ext;
+
+    (void) len;
+
+    ext = extension (fe->fname->str);
+
+    if (ext[0] == '\0' || S_ISDIR (fe->st.st_mode))
+        g_string_truncate (string_file_name_buffer, 0);
+    else
+        g_string_assign (string_file_name_buffer, ext);
+
+    return string_file_name_buffer->str;
+}
+
+/* --------------------------------------------------------------------------------------------- */
+
 static unsigned int
 ilog10 (dev_t n)
 {