Fix <memory.h> to be a synonym for <string.h> like in glibc.

Change-Id: If23589c5d85dffd28788e04b010303620fa178ca
libc: create sys/capability.h

Per "man capset", sys/capability.h is the appropriate header file
for the capget / capset definition, not unistd.h. Fixed.

As a short term hack, continue to include sys/capability.h in
unistd.h, until we can fix all the code which uses capget / capset.

Change-Id: I6e7cf55955d761ca785a14c5e4b7a44125d8fc15