Use more efficient implementation

Remove calling of map() because it is slow. Apply tolower() before
split().

Bench mark: https://gist.github.com/k-takata/b0647272fa5825b2c3eb
[WIP] Enable to specify mode characters to vimproc#fopen()
[WIP] Changed vimproc#fopen() caller argument
[WIP] Moved string-to-oflag convertion to C code
[WIP] Changed windows oflags; add leading underscore
[WIP] Modified vimproc#fopen help description
[WIP] Fixed missing variable
[WIP] Slightly changed
[WIP] Reverted close_allfd()