[libc] Rename pthread_impl.h to threads_impl.h

Baby step towards separating pthreads out of libc

Change-Id: Ibb1ba6656c796218909f4a3451db9bb66b99fb80

[musl] Remove internal signal handling code

Change-Id: I6e2c61766c41e2fcb21fda7a82aeb2ee52ad4afc

[musl][pthread] Remove pthread cancellation machinery

Nothing currently in Fuchsia uses this, nor am I aware of any 2nd or
3rd party code that is likely to. I do not believe that, if someone
were to attempt to truly implement pthread_cancel etc. on Magenta,
that the removal of this code and comments would be a meaningful
impediment.

Change-Id: Ib8435a68d0b053ba3f1d0590640ef54c23e59840

[musl] Reenable incompatible-pointer-types-discards-qualifiers for clang

Change-Id: Ib6ff47da98678e82695ad55dc5c8fc928bc7a70b

[musl] clang format musl

Change-Id: I3bc6ab7655410514691cd07f08454f3afd41d9b3

[magenta] Initial commit