Call posix_memalign() if building with `-mzos-target=zosv3r1` on z/OS 3.1,
in which case `__aligned_free()` calls `free()`. Otherwise use a local
implementation to align memory from `malloc()`.

On LE 3.1 zos-io.cc fails to pick up LOCK_* definitions from the
system's sys/file.h.

__clockid_t is defined in sys/types.h if >= v2r5,
CLOCK_REALTIME and CLOCK_MONOTONIC are already defined
in the system's time.h if >= v2r5.

Also remove unneeded include of features.h since other
system headers already include it.

This is for the local implementation to behave the same
way in terms of the argument values and return.