implementation of queuing of messages to send.
miniupnpd/asyncsendto: make sendto_schedule work
miniupnpd.c: improve comment for init()
miniupnpd.c: init random number generator in init()
miniupnpd/asyncsendto.h: improve comments/doc
asyncsendto: remove failed sendto() from queue
miniupnpd: add sendto() queuing
miniupnpd: use asyncsendto
minissdp.c: wait for a delay before answering M-SEARCH
miniupnpd/asyncsendto: finalize_sendto()
miniupnpd/minissdp.c: send ssdp:alive packets more than once

fixes #35
miniupnpd/minissdp: Introduce SSDP_RESPOND_SAME_VERSION

changes something when compiled as IGDv2.
Merge branch 'sendto_schedule'