EXECUTE: support for the third argument (quiet execution)

[slayoo]

As reported on SF.net: https://sourceforge.net/p/gnudatalanguage/feature-requests/80/

IDL> $cat ex3.pro
pro ex3sub
nonexistent
end
pro ex3, arg3
a = execute('ex3sub', 1, arg3)
end
IDL> ex3, 0
% Attempt to call undefined procedure/function: 'NONEXISTENT'.
IDL> ex3, 1
IDL> 

[slayoo]

Relevant info from SF/#56:

As reported by @alaingdl at SF.net (https://sourceforge.net/p/gnudatalanguage/bugs/692/):

two weakness in EXECUTE()

1/ the argument QuietCompile (now CompileFlags in IDL) is not well enforced
and has now 3 possible values (0,1, [2 : implied print])

1a/ quietCompile = p1->True(); is not OK (odd/even effect)
1b/ line 1553 is always verbose, even if flag asks to be quiet ...
RetCode retCode = caller->Interpreter()->execute( progAST);

e.g. : ok=EXECUTE("a = ABCDEF(/version)", flag) & print, ok
(assuming ABCDEF does not exist)
if flag=0 then message should appears (syntax error)
if flag != 0 then no message at all

2/ A new argument was added to EXECUTE(), I will (try to)
add it in the arg. number but it will not be enforced