Allow adding error symbols after early initialization

This was acomplished by making errSymbolAdd add the error symbol directly
into the global hash table and removing errnumlist which is not needed
anymore.

Unit tests were added for checking the following cases:
- Adding a valid symbol and checking that it exists (fixed by this change)
- Getting an existing error symbol
- Getting a non existing error symbol
- Adding an invalid error symbol (fixed by this change)
- Adding an error symbol with a code that already
  exists (fixed by this change)

Therefore, issue #268 was fixed

error: statically allocate error symbol hash table

This will allow calling errSymbolAdd before errSymBld, therefore, a
function adding error symbols can now be run before iocInit or errlogInit

error: add a constant for the minimum module number

Make adding an identical error symbol not fail

A test case was also added which test that adding an error symbol
with same error code and message as one added before will not fail

Add locking to error symbol table

This protects the cases of:
- simultaneously adding and requesting of an error symbol
- simultaneously adding many error symbols

Update release notes regarding adding error symbols

modules/libcom/src/error/errSymLib.c

@@ -23,7 +23,6 @@
 #include "cantProceed.h"
 #include "epicsAssert.h"
 #include "epicsStdio.h"
-#include "epicsTypes.h"
 #include "epicsMutex.h"
 #include "epicsThread.h"
 #include "errMdef.h"