include: posix: unistd: Fix prototypes and dependency

For read/write/lseek, use size_t and off_t types, as mandated by
POSIX:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/unistd.h.html



Also, prototypes of unistd.h functions should not depend on
CONFIG_POSIX_FS, as (many) of them deal with generic I/O, not with
files in filesystem per se.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>