[DEVEL] Improve socket options handling (remove redundant srt_post_opt_list structure)

[maxsharabayko]

There are two intersecting objects in SRT:

• srt_post_opt_list
• SrtOptionAction s_sockopt_action.

The first one (srt_post_opt_list) is only used by a group to check if an option can be used on a connected socket. But the same can be done using something like s_sockopt_action.flags[optname] & SRTO_POST.
There is no SRTO_POST at the moment, of course. The SRTO_POST_SPEC has a bit different purpose: to mark those POST options that need an additional special handling when set (e.g. updateCC).

The srt_post_opt_list seems to be redundant. s_sockopt_action.flags[optname] can be extended with SRTO_POST bit flags and then used by a group to detect POST options.

SRT versions: v1.5.3, v1.5.4.