Add support for setting UID/GID through mount options

[a-wai]

Hi,

The use-case is the following one:

• Mount an APFS read-only
• Create and overlayfs with the lower dir on the APFS directory and the upper dir on a user-writable partition
• The user cannot create a file in a subdirectory of the overlayfs as it is owned by root on the original filesystem with 755 permissions

Setting UID/GID for the whole APFS filesystem enables regular users writing to the overlayfs.

This patch solves issue #61

[a-wai]

Merged master to solve conflicts, feel free to let me know if you have any comments regarding this PR.

[nros]

The PR seems already merged with commit d9ae52ca4624bd2fb018c94206952d8214808050 So, this can be closed?