BLD: Add NPY_RELAXED_STRIDES_CHECKING flag

This flag will toggle to a new definition for the contiguous flags
where only the memory layout is forced to be correct. As a particular
example this means that np.ones((3,1)) can be both C and F-Contiguous
and its stride[-1] can be arbitrary but the array still contiguous.

The flag will also make most new arrays be created with MAX_INTP as
stride so that unsafe usage of the stride will surface more commonly.
ENH: Relax aligned flag for shape[i] <= 1

In this case, either the dimensions stride will never be used to
access an element, so that it does not matter to the data alignment,
or the array has a size of 0 and is thus never unaligned.

Relaxed align flag is only active if NPY_RELAXED_STRIDES_CHECKING
was set during compile time.
API: Implement new flags API with NPY_RELAXED_STRIDES_CHECKING

This largly reverts the changes to the flags setting api making the
newer preferable unsafe flags setting available through the
NPY_RELAXED_STRIDES_CHECKING eviroment variable. This variable is meant
for testing if code will stop working when the flags definition is changed.

The old definition is modified in some details to previously to enforce
safer strides (which was not the case before). This means that ndim==1
size==1 arrays are not necessarily considered contiguous. Also empty
arrays are not considered contiguous in some cases that were contiguous
before, and the rule that an array can only be both C and F-contiguous
if it is one (or zero) dimensional is relaxed, as it is incorrect for
size <= 1 arrays.
TST: Add NPY_RELAXED_STRIDES_CHECKING to Travis
Merge pull request #3162 from seberg/unsafe-strides

Revert changes to contiguous flags definition while creating NPY_TEST_UNSAFE_STRIDES