bgpd: fix SIGBUS
There is one test failure in the testsuite on sparc:
Running ./bgpd.tests/testbgpcap.exp ...
failed: testbgpcap ORF: ORF, simple, single entry, single tuple -- testbgpcap aborted!
The error is a SIGBUS in bgp_capability_mp_data() because of an unaligned
memory access. Use memcpy() instead of direct assignments. Compilers on
platforms that support unaligned accesses should be clever enough to
optimize the function call away and do the direct store, so this should not