basic/missing: move syscall definitions to basic/missing_syscall.h

basic/missing: move syscall definitions to basic/missing_syscall.h

We have a bunch of syscall wrapper definitions and it's easier to
see that they follow the same pattern if they are not interspersed
with other defines.

Change the wrappers to be uniform:
- if __NR_XXX is not defined, do not bother to call the syscall,
  and return -1/ENOSYS immediately.
- do not check __NR_XXX defines if we detect the symbol as defined,
  since we don't need them anyway
- reindent stuff for readability

New file basic/missing_syscall.h is included at the end of missing.h
because it might make use of some of the definitions in missing.h.