Port basic functionality in libltp to FreeBSD

This involved...

- Muting a clang warnings
- Removing #includes to malloc.h
- Stubbing out Linux-specific pieces.
- Stubbing out memalign (it's an obsolete Linux syscall)
- Fix typecasting in va_arg in safe_open function to mute
  clang warnings
- Use SIGCHLD instead of SIGCLD for portability reasons
- Port errnos.h to FreeBSD
- Remove sys/sysmacros.h (it was unused)
- Port tst_resm to portable pthreads

Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Sponsored-by: EMC / Isilon Storage Division
Port the mmapstress test code to FreeBSD

Sponsored-by: EMC / Isilon Storage Division
Signed-off-by: Garrett Cooper <yanegomi@gmail.com>