Copy cmemcpy.h to libcheri, and install with other CHERI includes,
since cmemcpy() is now defined in libcheri (and libc_cheri).
Coalesce various instances of 'cmemcpy.h' to 'cheri/cheri_memcpy.h'.  Fix
tcpdump build by giving it access to the header, which might have been
missed in an earlier commit.