lib: utils: Have parse_one_of() warn about prefix matches

lib: utils: Have parse_one_of() warn about prefix matches

The function parse_one_of() currently uses matches() for string comparison
under the hood. Extending matches()-based parsers is tricky, because newly
added matches might change the way strings are parsed, if the newly-added
string shares a prefix with a string that is matched later in the code.

Therefore in this patch, add a twist to parse_one_of() that partial prefix
matches yield a warning. This will not disturb standard output or the
overall behavior, but will make it obvious that the usage is discouraged
and prompt users to update their scripts and habits.

An example of output:

    # dcb ets set dev swp1 tc-tsa 0:s
    WARNING: 's' matches 'strict' by prefix.
    Matching by prefix is deprecated in this context, please use the full string.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>