Make tpm_emulator-0.7.4 use /dev/tpm_emul

This patch makes tpm_emulator-0.7.4 use /dev/tpm_emul instead of a socket.
Clients would then access the emulator via /dev/tpmN.

It would be possible to make the select loop poll the /dev/tpm_emul and
support both /dev/tpm_emul and the socket simultaneously.