Avoid race conditions when creating directories

[ajhool]

Added a mkdirSync function to libvips that gracefully handles existing directories.

Chose to leave the name as mkdirSync because the functionality is almost identical to fs.mkdirSync and a name like "safeMakeDirSync" might overstate the scope of this function. Definitely open to suggestion, though.

[coveralls]

Coverage increased (+0.1%) to 98.91% when pulling 5485226 on ajhool:ensureDir into 5bed3a7 on lovell:master.

[lovell]

Fantastic, thank you!