Do not enforce minimum alignment in memalign().

Do not enforce minimum alignment in memalign().  This is a non-standard
function, and there is disagreement over whether to enforce minimum
alignment.  Solaris documentation (whence memalign() originated) says
that minimum alignment is required:

  The value of alignment must be a power of two and must be greater than
  or equal  to  the size of a word.

However, Linux's manual page says in its NOTES section:

  memalign() may not check that the boundary parameter is correct.

This is descriptive rather than prescriptive, but applications with
bad assumptions about memalign() exist, so be as forgiving as possible.

Reported by Mike Hommey.