ime: fcitx: use SDL_GetExeName() in GetAppName()

Use the existing SDL_GetExeName(), available for all the UNIX
platforms, in the internal GetAppName(); this has few advantanges:
- SDL_GetExeName() (and SDL_GetAppID() that builds on top of it) are
  used in various places already; since it caches the executable name,
  this may remove one extra read of the application name
- SDL_GetExeName() has a non-dummy implementation in more OSes than
  GetAppName(), thus providing a small improvement for this IME

As drive-by change: since SDL_GetExeName() provides a constant string,
there is no more need to allocate a new string in GetAppName(), which
is used as constant string anyway. Hence, return a constant string in
GetAppName() too.

(cherry picked from commit 248bcf6b29de94e9f1d6e82e75613ec672fe851c)

src/core/linux/SDL_fcitx.c

@@ -25,6 +25,7 @@
 #include "SDL_fcitx.h"
 #include "../../video/SDL_sysvideo.h"
 #include "../../events/SDL_keyboard_c.h"
+#include "../../core/unix/SDL_appid.h"
 #include "SDL_dbus.h"
 
 #ifdef SDL_VIDEO_DRIVER_X11
@@ -53,32 +54,14 @@ typedef struct FcitxClient
 
 static FcitxClient fcitx_client;
 
-static char *GetAppName(void)
+static const char *GetAppName(void)
 {
-#if defined(SDL_PLATFORM_LINUX) || defined(SDL_PLATFORM_FREEBSD)
-    char *spot;
-    char procfile[1024];
-    char linkfile[1024];
-    int linksize;
-
-#ifdef SDL_PLATFORM_LINUX
-    (void)SDL_snprintf(procfile, sizeof(procfile), "/proc/%d/exe", getpid());
-#elif defined(SDL_PLATFORM_FREEBSD)
-    (void)SDL_snprintf(procfile, sizeof(procfile), "/proc/%d/file", getpid());
-#endif
-    linksize = readlink(procfile, linkfile, sizeof(linkfile) - 1);
-    if (linksize > 0) {
-        linkfile[linksize] = '\0';
-        spot = SDL_strrchr(linkfile, '/');
-        if (spot) {
-            return SDL_strdup(spot + 1);
-        } else {
-            return SDL_strdup(linkfile);
-        }
+    const char *exe_name = SDL_GetExeName();
+    if (exe_name) {
+        return exe_name;
     }
-#endif // SDL_PLATFORM_LINUX || SDL_PLATFORM_FREEBSD
 
-    return SDL_strdup("SDL_App");
+    return "SDL_App";
 }
 
 static size_t Fcitx_GetPreeditString(SDL_DBusContext *dbus,
@@ -281,7 +264,7 @@ static bool FcitxCreateInputContext(SDL_DBusContext *dbus, const char *appname,
 
 static bool FcitxClientCreateIC(FcitxClient *client)
 {
-    char *appname = GetAppName();
+    const char *appname = GetAppName();
     char *ic_path = NULL;
     SDL_DBusContext *dbus = client->dbus;
 
@@ -290,8 +273,6 @@ static bool FcitxClientCreateIC(FcitxClient *client)
         ic_path = NULL; // just in case.
     }
 
-    SDL_free(appname);
-
     if (ic_path) {
         SDL_free(client->ic_path);
         client->ic_path = SDL_strdup(ic_path);