sceIo: stop clobbering st_private, report FAT permissions, implement sceIoChstat

Cashing in the io/stat and io/shortname recordings.

__IoGetStat began with memset(stat, 0xfe, sizeof(SceIoStat)), which destroyed 24 bytes of the
caller's buffer that a real PSP never touches - it writes only as far as the timestamps and
leaves all six st_private words exactly as it found them. It also wrote a made-up sector number
into st_private[0] on the memory stick. That word carries the LBN on a UMD, which games read to
build disc0:/sce_lbn paths, so it stays for non-FAT and is left alone otherwise.

FAT has no permissions of its own and everything reads back as 0777. We were passing the host's
idea of the file through instead. The existing "all files look executable on FAT" hack for Beats
(issue #14812) was right in substance but lived only in sceIoDread, so sceIoGetstat and
sceIoDread disagreed about the same file where hardware has them agree. Both now go through one
path, which also gets the read-only case right: no write bits means mode 0555 and attr 0x21.

sceIoGetstat on the root of a volume is refused, as on hardware.

sceIoChstat was a logging stub. It now applies the read-only flag, which is what st_mode's write
bits and st_attr's 0x01 both mean on FAT - setting either produces both, and it's reversible.
That needs a new IFileSystem::SetFileWritable, defaulting to "can't" so read-only filesystems and
hosts that can't express it (Android content URIs) are unaffected; the call still succeeds there,
since hardware would have.

GenerateFatShortNames now accounts for capitalisation. FAT keeps a lowercase flag for the base and
another for the extension, but the PSP only honours the base one, so "shrt" becomes SHRT while
"readme.txt" becomes README~1.TXT. We were only adding a counter on collision. The unit test
carries the whole recorded set, including the corrected README~1.MD.

io/shortname stays in tests_next: its d_name column can't match while SimulateVFATBug is
uppercasing lowercase 8.3 names, which is deliberate and load-bearing for homebrew.

Core/FileSystems/MetaFileSystem.cpp

@@ -507,6 +507,19 @@ int MetaFileSystem::RenameFile(const std::string &from, const std::string &to)
 	}
 }
 
+bool MetaFileSystem::SetFileWritable(const std::string &filename, bool writable)
+{
+	std::lock_guard<std::recursive_mutex> guard(lock);
+	std::string of;
+	IFileSystem *system;
+	int error = MapFilePath(filename, &of, &system);
+	if (error == 0) {
+		return system->SetFileWritable(of, writable);
+	} else {
+		return false;
+	}
+}
+
 bool MetaFileSystem::RemoveFile(const std::string &filename)
 {
 	std::lock_guard<std::recursive_mutex> guard(lock);