high-level: add fmask and dmask options

high-level: add fmask and dmask options

dmask: umask applied to directories
fmask: umask applied to non-directories

to get "typical" permission bits for regular files (0644) and directories (0755), a single
umask option is not sufficient (or well, it isn't the way fuse implements it)

there is precident for separate umask and dmask options in other
filesystems (see for example fat: https://github.com/torvalds/linux/tree/master/fs/fat)

this addition should not affect backward-compatibility; the original
umask option retains the same meaning, but non-zero fmask or
dmask will override it.