Add more signal_set_base::flags_t constants

[vinipsmaker]

This commit only adds useful POSIX constants that don't conflict with asio's signal handling strategy (e.g. SA_SIGINFO would break asio interface, so it's avoided). Non-POSIX constants specific to certain operating systems (e.g. Linux's SA_UNSUPPORTED) are ignored.

[vinipsmaker]

Alternatively, you could use no_child_stop instead of nocldstop and no_child_wait instead of nocldwait, but I can't know what style you prefer, so you'll have to choose yourself.

[vinipsmaker]

Landed in master: https://github.com/chriskohlhoff/asio/commit/11c6cafd3d4397fedb37003f119506a296813044