awk: Fix overly permissive func arg list parsing

awk: Fix overly permissive func arg list parsing

It allows things like 'func f(a b)' and 'func f(a,)' which GNU awk forbids.

function                                             old     new   delta
parse_program                                        327     367     +40
chain_expr                                            40      67     +27
parse_expr                                           891     915     +24
EMSG_TOO_FEW_ARGS                                     30      18     -12
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/1 up/down: 91/-12)             Total: 79 bytes

Signed-off-by: Brian Foley <bpfoley@google.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>