[server] Fix uninitialized value usage after proper SPA authentication/decryption

Bug fix discovered with the libfiu fault injection tag
"fko_get_username_init" combined with valgrind analysis. This bug
is only triggered after a valid authenticated and decrypted SPA
packet is sniffed by fwknopd:

==11181== Conditional jump or move depends on uninitialised value(s)
==11181==    at 0x113B6D: incoming_spa (incoming_spa.c:707)
==11181==    by 0x11559F: process_packet (process_packet.c:211)
==11181==    by 0x5270857: ??? (in /usr/lib/x86_64-linux-gnu/libpcap.so.1.4.0)
==11181==    by 0x114BCC: pcap_capture (pcap_capture.c:270)
==11181==    by 0x10F32C: main (fwknopd.c:195)
==11181==  Uninitialised value was created by a stack allocation
==11181==    at 0x113476: incoming_spa (incoming_spa.c:294)
[server] refactor main() into a more natural breakdown of functions
[test suite] add SYSLOG_FACILITY tests