Fix symlink read-/write-through code

Use `realpath(3)` instead of `readlink(3)`, which offers support for
relative and absolute symlinks alike, since all symbolic links will be
resolved to the absolute path of the linked-to file.

Fixes #8082

eeschema/sch_plugins/kicad/sch_sexpr_plugin.cpp

@@ -1384,15 +1384,11 @@ wxFileName SCH_SEXPR_PLUGIN_CACHE::GetRealFile() const
 #ifndef __WINDOWS__
     if( fn.Exists( wxFILE_EXISTS_SYMLINK ) )
     {
-        char buffer[ PATH_MAX + 1 ];
-        ssize_t pathLen = readlink( TO_UTF8( fn.GetFullPath() ), buffer, PATH_MAX );
+        char buffer[ PATH_MAX ];
+        char *realPath = realpath( TO_UTF8( fn.GetFullPath() ), buffer );
 
-        if( pathLen > 0 )
-        {
-            buffer[ pathLen ] = '\0';
-            fn.Assign( fn.GetPath() + wxT( "/" ) + wxString::FromUTF8( buffer ) );
-            fn.Normalize();
-        }
+        if( realPath )
+            fn.Assign( wxString::FromUTF8( realPath ) );
     }
 #endif
 

eeschema/sch_plugins/legacy/sch_legacy_plugin.cpp

@@ -2441,15 +2441,11 @@ wxFileName SCH_LEGACY_PLUGIN_CACHE::GetRealFile() const
 #ifndef __WINDOWS__
     if( fn.Exists( wxFILE_EXISTS_SYMLINK ) )
     {
-        char buffer[ PATH_MAX + 1 ];
-        ssize_t pathLen = readlink( TO_UTF8( fn.GetFullPath() ), buffer, PATH_MAX );
+        char buffer[ PATH_MAX ];
+        char *realPath = realpath( TO_UTF8( fn.GetFullPath() ), buffer );
 
-        if( pathLen > 0 )
-        {
-            buffer[ pathLen ] = '\0';
-            fn.Assign( fn.GetPath() + wxT( "/" ) + wxString::FromUTF8( buffer ) );
-            fn.Normalize();
-        }
+        if( realPath )
+            fn.Assign( wxString::FromUTF8( realPath ) );
     }
 #endif
 

pcbnew/plugins/kicad/kicad_plugin.cpp

@@ -2409,15 +2409,11 @@ void PCB_IO::FootprintSave( const wxString& aLibraryPath, const FOOTPRINT* aFoot
     // Write through symlinks, don't replace them
     if( fn.Exists( wxFILE_EXISTS_SYMLINK ) )
     {
-        char buffer[ PATH_MAX + 1 ];
-        ssize_t pathLen = readlink( TO_UTF8( fn.GetFullPath() ), buffer, PATH_MAX );
+        char buffer[ PATH_MAX ];
+        char *realPath = realpath( TO_UTF8( fn.GetFullPath() ), buffer );
 
-        if( pathLen > 0 )
-        {
-            buffer[ pathLen ] = '\0';
-            fn.Assign( fn.GetPath() + wxT( "/" ) + wxString::FromUTF8( buffer ) );
-            fn.Normalize();
-        }
+        if( realPath )
+            fn.Assign( wxString::FromUTF8( realPath ) );
     }
 #endif