Instance: lock the shared counter for real, and don't hand out ID 0

The POSIX path used mlock() as though it were a mutex. mlock only pins pages in
RAM - it provides no mutual exclusion at all, so the read-modify-write of the
cross-process instance counter was unsynchronized. Two instances launched at the
same moment could both come away with PPSSPP_ID == 1, at which point both pass
IsFirstInstance() and write ppsspp.ini over each other, and both compute the same
adhoc local IP. Take an advisory lock on the shm fd instead. (The Windows path was
already fine - it uses a named mutex.)

Also, next/total are uint8_t in a segment that outlives the processes using it, so
next climbs across runs and wraps. Landing on 0 is worse than it looks: it isn't a
valid instance id, IsFirstInstance() fails, and config saving is silently disabled
from then on. Skip past it on wrap.

Core/Instance.cpp

@@ -22,6 +22,7 @@
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/mman.h>
+#include <sys/file.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #endif
@@ -96,10 +97,16 @@ static bool UpdateInstanceCounter(void (*callback)(volatile InstanceInfo *)) {
 	}
 
 	bool result = false;
-	if (mlock(buf, BUF_SIZE) == 0) {
+	// An actual advisory lock on the shm object. This used to call mlock(), which only pins
+	// pages in RAM and provides no mutual exclusion whatsoever - two instances starting at the
+	// same moment could both read the counter and come away with the same PPSSPP_ID, then both
+	// believe they were the first instance and write the config over each other.
+	if (flock(hIDMapFile, LOCK_EX) == 0) {
 		callback(buf);
-		munlock(buf, BUF_SIZE);
+		flock(hIDMapFile, LOCK_UN);
 		result = true;
+	} else {
+		ERROR_LOG(Log::sceNet, "flock(%s) failure: %s", ID_SHM_NAME, GetLastErrorMsg().c_str());
 	}
 
 	munmap(buf, BUF_SIZE);
@@ -157,7 +164,14 @@ void InitInstanceCounter() {
 #endif
 
 	bool success = UpdateInstanceCounter([](volatile InstanceInfo *buf) {
-		PPSSPP_ID = ++buf->next;
+		// The shared segment outlives the processes that used it (see the shm_unlink comment),
+		// so next keeps climbing across runs and eventually wraps this uint8_t. ID 0 is not a
+		// valid instance - it fails the IsFirstInstance() check, which quietly disables config
+		// saving - so skip past it.
+		if (++buf->next == 0) {
+			buf->next = 1;
+		}
+		PPSSPP_ID = buf->next;
 		buf->total++;
 	});
 	if (!success) {