Support systems without posix_memalign() and memalign()

Support systems without posix_memalign() and memalign()

MacOS 10.5 doesn't have posix_memalign() nor memalign(), but it does
have valloc().  The Android SDK would like to be built on MacOS 10.5,
so I've added support for a good-enough emulation of memalign()'s
functionality using valloc(), with an explicit test to make sure
valloc() is returning a pointer which is sufficiently aligned given
the requested alignment.  This won't work if you try to operate on a
file system with a 16k blocksize using an e2fsprogs built on MacOS
10.5 system, but it is good enough for the common case of 4k
blocksize file systems, and we will let the memory allocation fail in
the alignment is not good enough.

I've also added a unit test for ext2fs_get_memalign() so we can be
sure it's working as expected.  I've tested the code paths with
HAVE_POSIX_MEMALIGN defined, HAVE_POSIX_MEMALIGN undefined, and
HAVE_POSIX_MEMALIGN and HAVE_MEMALIGN undefined on an x86 Linux
system, and so I know the valloc() code path works OK.  The simplistic
(and less safe) patch at:

https://trac.macports.org/attachment/ticket/33692/patch-lib-ext2fs-inline.c.diff

Shows that using valloc() apparently works OK for MacOS 10.5 (but if
it doesn't the unit test will catch a problem).

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>