[server] Added FORCE_SNAT to access.conf stanzas.

Added FORCE_SNAT to the access.conf file so that per-access stanza SNAT
criteria can be specified for SPA access.
[libfko] added defensive NULL check for is_valid_ipv4_addr()
[server] added the ability to use FORCE_MASQUERADE to access.conf stanzas
[test suite] added missing config files
[test suite] added masquerade exception for non-Linux systems