Cleanup on aisle 5: normalizes the Lua code to follow common schema/model

Cleanup on aisle 5: normalizes the Lua code to follow common schema/model

Over time the various wslua classes/functions have gotten moldy, with different
ways of doing similar things. Some of it can't be changed without breaking
backwards compatibility for Lua scripts, so I didn't do that. But I did what
I could. The biggest change is a refactoring of how accessors/attributes
are handled in the code, so that most of them work the same way using the
same code.

Specific changes made:
 * Added null/expired checking macro to class declarations for many classes
 * Removed extraneous pointer/expired checking, since checkFoo() does that already
 * Fixed "errors" reported by clang static analyzer; they were false positives, but it was easier to get it to stop complaining by changing the code
 * Moved internal wslua functions from wslua_utils.c into a new 'wslua_internals.c' file
 * Changed Listener/NSTime/Pinfo/Proto to use a common setter/getter accessor/attribute code model, instead of each of them doing their own
 * Fixed some API doc mistakes, mostly around attributes that were documented as read-only but were actually read-write

Change-Id: Idddafc5fbd3545ebff29e063acc767e1c743a1a9
Reviewed-on: https://code.wireshark.org/review/271
Reviewed-by: Evan Huus <eapache@gmail.com>
Tested-by: Evan Huus <eapache@gmail.com>