elm_datetime: Add 'O' character to ignore POSIX alternative format in format.

elm_datetime: Add 'O' character to ignore POSIX alternative format in format.

Summary:
_parse_format() function filtering "_/-/0/^/*" to filter POSIX alternatvie/extension formats,
But the Datetime do not appear in some locale(fa_IR, lzh_TW, my_MM, or_IN) with 'O'.

+ improve code. (not compare with all of the characters. instead of that using strchr)

https://lh.2xlibre.net/values/d_t_fmt/

@fix

Test Plan: Set locale as fa_IR and create DateTime. and see fields, not appears.

Reviewers: jpeg, cedric

Subscribers: cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D5020

Change-Id: I5c64f301324834b771c452c9cf39da6faa782274