Fix IO#reopen, add open3 missing sigs

[HoneyryderChuck]

IO#reopen should be able to receive the same kwargs that IO#open receives; also, the mode can be an integer too.

Have a separate commit adding missing sigs for open3. @soutaro this is currently a WIP, as the sigs can't be expressed fully in RBS:

• most functions accept an optional options hash at the end, despite commands (string) already supporting optional (string) args as well. it's like a second set of optional args, and this can't be expressed in RBS.
• some functions (i.e. pipeline_w return an array where the first element is stdin/stdout (IO) and the rest is one or more wait threads. "variable size" tuples can't be expressed in RBS yet as well.

[HoneyryderChuck]

@ksss I'll write the tests once we address the broken behaviour for this type of signature 👍