[os_maild] use flag directly
[os_maild] move include directive
[os_maild] remove unused struct member
[os_maild] replace allocation code with macro and use malloc instead of calloc, cause we are initializing all members
[os_maild] remove variable
[os_maild] rename variable for readability
[os_maild] directly pass flag, reduce variable scope
[os_maild] change variable type to original type
[os_maild] add const qualifier