Back-patch assorted latch-related fixes.

Fix a whole bunch of signal handlers that had been hacked to do things that
might change errno, without adding the necessary save/restore logic for
errno.  Also make some minor fixes in unix_latch.c, and clean up bizarre
and unsafe scheme for disowning the process's latch.  While at it, rename
the PGPROC latch field to procLatch for consistency with 9.2.

Issues noted while reviewing a patch by Peter Geoghegan.

src/include/replication/syncrep.h

@@ -33,8 +33,8 @@ extern char *SyncRepStandbyNames;
 /* called by user backend */
 extern void SyncRepWaitForLSN(XLogRecPtr XactCommitLSN);
 
-/* callback at backend exit */
-extern void SyncRepCleanupAtProcExit(int code, Datum arg);
+/* called at backend exit */
+extern void SyncRepCleanupAtProcExit(void);
 
 /* called by wal sender */
 extern void SyncRepInitConfig(void);