cql_repl: use POSIX primitives to reset input/output
cql_repl: use POSIX primitives to reset input/output

Seastar uses POSIX IO for output in addition to C++ iostreams,
e.g. in print_safe(), where it write()s directly to stdout.

Instead of manipulating C++ output streams to reset
stdout/log files, reopen the underlying file descriptors
to output/log files.

Fixes #9962 "cql_repl prints junk into the log"
Message-Id: <0095a0d358120cc3d30c242b93c8156bc6b74d92@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla