Fixed -hidden on hidden start paths

Added a Bash completion script. Thanks @bmundt6!

Fixed rounding in -used. Corresponding fixes were made to GNU find in version 4.8.0.

Optimized the open directory representation. On Linux, much libc overhead is bypassed by issuing syscalls directly. On all platforms, a few fewer syscalls and open file descriptors will be used.

Implemented -flags from BSD find