ss: Review ssfilter

ss: Review ssfilter

The original problem was ssfilter rejecting single expressions if
enclosed in braces, such as:

| sport = 22 or ( dport = 22 )

This is fixed by allowing 'expr' to be an 'exprlist' enclosed in braces.
The no longer required recursion in 'exprlist' being an 'exprlist'
enclosed in braces is dropped.

In addition to that, a few other things are changed:

* Remove pointless 'null' prefix in 'appled' before 'exprlist'.
* For simple equals matches, '=' operator was required for ports but not
  allowed for hosts. Make this consistent by making '=' operator
  optional in both cases.

Reported-by: Samuel Mannehed <samuel@cendio.se>
Fixes: b2038cc0b2403 ("ssfilter: Eliminate shift/reduce conflicts")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>