parse: use minimum delimiter distance

parse: use minimum delimiter distance

Use minimal distance to delimiter to determine option length

Current implementation of opt_len() makes impossible to
locate option name in random_distribution zones list
combining ':' and ',' chars.
opt_len() function should try to locate option name
by all possible delimiters and return minimal length one
instead of returning first found.

Fixes: https://github.com/axboe/fio/issues/1923

Signed-off-by: Leonid Kozlov <leonid.e.kozlov@gmail.com>