Currently a buffer with a fixed size of 8192 bytes is used. However the
result can potentially be larger, which leads to a truncated JSON
output on stdout. UDP packets, without compression and with IP
fragmentation, can be up to 64KiB large.

Instead of using a fixed size buffer on the stack ask the kernel first
about the size of the UDP data and allocate a buffer of appropriate size
on the heap before receiving the UDP data.

The issue was observed with a custom respondd provider.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>

End the process after one result in case -l is not given
and destination address is unicast.
Reduces singleshot execution time from timeout seconds to around 150ms.

resolves #2184

inet_pton() doesn't set errno on regular parse errors, so using perror()
doesn't make sense (and we avoid a confusing "Success" message).

gluon-neighbour-info: improve verbosity on wrong cli parameters

The timeout was calculated incorrectly (in each iteration the span between
the start time and the current time was substracted from the timeout
again), and would often become negative, causing warnings in the kernel
log.

sse-multiplex likes to get the header as fast as possible.

error() is a glibc specific function and should not be used in
code meant to be portable.

This is a simple client for gluon-announced.