Fix "pselect: Use linux pselect6 syscall when available"

Commit e3c3bf2b58 introduce use of pselect6, but has following disadvantages:

* Use of userspace types in args67 structure - it breaks, for example,
  configs when 32-bit uClibc-ng compiled against 64-bit kernel. Syscall
  will always return EINVAL. We must use __kernel_* types and
  __SYSCALL_SIGSET_T_SIZE.

* It have excess checks for NSEC_PER_SEC. Original code from select()
  implementation has struct timeval => struct timespec conversion,
  kernel select() syscall implementation do the same.
  But none of libc versions (glibc, eglibc, musl) I know, perform similar
  checks for pselect() - there is no structure fields conversions,
  just struct timespec through all the calls.
  To have such checks in uClibc-ng we need one example, at least.

* It is possible to avoid extra userspace reads from kernel code if
  sigmask == NULL. I suggest to do it, for a few bytes cost.

* Commit didn't add test case to testsuite.

Signed-off-by: Leonid Lisovskiy <lly.dev@gmail.com>

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