samples: net: sockets: Clean up/improve socket samples

With 323e8cf0 applied and printf() working out of the box,
CONFIG_NET_BUF_LOG=y workarounds can be removed from configs of
all samples.

Also, print an intro message at the start of each server sample,
to give a user hint that the app didn't just hang and what to do
next. (The port waiting for connection is printed. We can't (easily)
print host address, because the samples should run on both Zephyr
and POSIX systems, and finding out local host address would require
hairy #ifdef's undermining the purpose of these samples (that is,
showing that the *same* code can be used on both types of systems)).

Fixes: #5379

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>