Split posix_spawn.c into 3 different files

posix_spawnattr.c
posix_spawn_file_actions.c
posix_spawn.c

Also, move the declarations of the posix_spawn types
into spawn.h.

These two changes combined allow alternative
implementations of posix_spawn to be provided without
duplicating the posix_spawnattr* and
posix_spawn_file_actions* functions.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3976
R=phosek@chromium.org

Review URL: https://codereview.chromium.org/650343002
6 files changed