add tpc/bin/tcpperf.cc

Compile tpc on FreeBSD 13.

Use TEMP_FAILURE_RETRY() to deal with EINTR.

Also replace read(2) with recv(2), write(2) with send(2).
On Linux 5.10, recv/send save a hop in VFS, so it's a bit more
efficient.

read -> do_syscall_64 -> ksys_read -> vfs_read -> new_sync_read
-> call_read_iter -> sock_read_iter -> inet_recvmsg
readv -> do_syscall_64 -> do_readv -> vfs_readv -> do_iter_read
-> do_iter_readv_writev -> call_read_iter -> sock_read_iter
-> inet_recvmsg
recv -> do_syscall_64 -> __x64_sys_recvfrom -> __se_sys_recvfrom
-> __do_sys_recvfrom -> __sys_recv_from -> inet_recvmsg

write -> do_syscall_64 -> ksys_write -> vfs_write -> new_sync_write
-> call_write_iter -> sock_write_iter -> sock_sendmsg
send -> do_syscall_64 -> __x64_sys_sendto -> __se_sys_sendto
-> __do_sys_sendto -> __sys_sendto -> sock_sendmsg

Support IPv6.

add bin/footprint

add ssl/TlsStream etc.

organize source files in tpc/