Make SOC services thread-safe

[ZeroSkill1]

As it stands, the soc:U client-side implementation in libctru will break when used across more than one thread. This is because while one thread is sending a soc:U command, all others would be blocked if they use those commands until that thread is finished with it.

Official system modules such as friends and http allocate a pool of soc:U handles and implement a "give/take" system, where handles are popped from the pool, used for a command, then reinserted afterwards.

I have implemented a similar kind of handle pool, with backwards compatibility by making the usual socInit initialize only one session, and implementing a new function socInitMulti to provide the user with control for how many sessions should be allocated depending on the use case.

I've tested this change to be working with:

• this sample program for multi threaded usage, and
• this sample program for single threaded usage, for backwards compatibility.