Add PAM_RUSER format specificier

[msantos]

Add a new "%r" format specifier to represent the PAM remote user:

auth sufficient pam_ssh_agent_auth.so file=/etc/keys/%r

This change allows programs, like su, that set RUSER to the origin user and USER to the destination user, to name the key according to the user calling the program:

# auser -> root
/etc/keys/%r # equivalent to /etc/keys/auser
/etc/keys/%u # equivalent to /etc/keys/root

[msantos]

specificier -> specifier