Add <sys/cpumask.h>.

Add <sys/cpumask.h>.

 Collect and gather all scatter cpumask bits to correct headers. This
 cleans up the namespace and simplifies platform handling in asm macros.
 The cpumask_t together with its macros is already non MI feature that is
 used in userland utilities, libraries, kernel scheduler and syscalls.
 It deserves sys/ header.  Adjust syscalls.master and rerun sysent.

 While there, fix an issue in ports that set POSIX env, but has
 implementation of setting thread names through pthread_set_name_np().