petscsys.h: find memcpy and memset in string.h

C89 and later say that memcpy and memset are in string.h.  With glibc,
memory.h merely includes string.h, and one user reported an error due to
undefined memset (which I cannot reproduce).  So quit bothering with
nonstandard memory.h and just include string.h.

Reported-by: Alexander Krimm <alex@wirew0rm.de>
VecView_MPI_DA: fix gsizes bug (bad conversion in parent commit)

Reported-by: Xiangdong <epscodes@gmail.com>
Merge commit 'VecView_MPI_DA: fix gsizes bug (bad conversion in parent commit)'
dense: fix overflow with int32*int32 during malloc()
Increase patchlevel to 3.4.4
DTGaussQuadrature: fix weight averaging for npoints=2 and output
Merge branch 'jed/fix-memcpy-in-string.h' into maint

* jed/fix-memcpy-in-string.h:
  petscsys.h: find memcpy and memset in string.h