Call fdatasync/fsync after writing out non-meta pages

This avoids a case where writes can be reordered so meta page is
written before a page it refers to, potentially causing a corrupt
database after a power loss or kernel crash.
Merge branch 'fdatasync' of https://github.com/tv42/bolt into fsync
Consolidate syscall files.
Merge pull request #76 from benbjohnson/fsync

fdatasync