Process: Use s:TYPE_* constants instead of type()
Process: system(): Fix conversion (#169)

The followings need conversion unconditionally.
* system()'s {input} (the 2nd argument)

Commit df94272 was incomplete.
Process: system(): Fix duplicate {command}

{command} [, {input} [, {timeout}]]
Process: system(): Process optional arguments at first

Process optional arguments at first
because use_vimproc is required later for a:str argument.