samples: net: sockets: echo_client/server: Bump ZVFS_OPEN_MAX config

Recent POSIX changes caused that 3 file descriptors are now preallocated
for stdin/out/err. This caused file descriptor shortage in all-in TLS
configuration of the sample, hence increase the maximum FD count.

In the server sample this manifested itself as an accept() error. This
triggered a busy loop though in the sample, as in case of accept()
errors it'd just try again w/o any delay. This made this issue hard to
investigate, so to avoid such cases in the future, make the accept()
failure fatal in the echo_server sample.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>