Add socket based server

[ueno]

This pull request adds a new p11-kit server command, which provides a Unix domain socket based server to expose access to smartcards.

[ueno]

I have added the stuff needed for PKCS#11 forwarding through OpenSSH. Here is an example usage.

On the local host, start the server:

local$ p11-kit server 'pkcs11:model=SoftHSM%20v2;manufacturer=SoftHSM%20project;serial=67060e945183d131;token=Daiki%27s%20token'
P11_KIT_SERVER_ADDRESS='/run/user/15082/p11-kit/pkcs11-12345'
P11_KIT_SERVER_PID=12345

and forward the socket with ssh -R:

local$ ssh -R /run/user/<remote-user-id>/p11-kit/pkcs11:$P11_KIT_SERVER_ADDRESS remote

Note that /run/user/<remote-user-id>/p11-kit must be created in advance. One way to automate that is using systemd user service:

remote$ cat ~/.config/systemd/user/p11-kit-client.service
[Unit]
Description=Create directory for p11-kit client
[Service]
Type=oneshot
ExecStart=/usr/bin/mkdir %t/p11-kit
[Install]
WantedBy=default.target

remote$ systemctl --user enable p11-kit-client.service

On the remote host, access the forwarded socket through p11-kit-client.so:

remote$ p11tool --provider /usr/lib64/pkcs11/p11-kit-client.so --test-sign --login 'pkcs11:model=SoftHSM%20v2;manufacturer=SoftHSM%20project;serial=67060e945183d131;token=Daiki%27s%20token;id=%09%7c%5e%bd%55%a6%37%d7%ee%99%e1%f9%7e%71%cf%56%81%8d%60%9f;object=Daiki%27s%20key;type=private'
Token 'Daiki's token' with URL 'pkcs11:model=SoftHSM%20v2;manufacturer=SoftHSM%20project;serial=67060e945183d131;token=Daiki%27s%20token' requires user PIN
Enter PIN: 
Verifying against private key parameters... ok
Verifying against public key in the token... ok

[ueno]

Rebased against #38