add support for fsflags= option in twrp.fstab file

example userdata line (as needed by MotoX):
/data         f2fs      /dev/block/platform/msm_sdcc.1/by-name/userdata       flags=fsflags="inline_xattr"

Code for parsing flags/options originally based on AOSP fs_mgr sources.

Change-Id: I5fb2b5d5cdd08137e6bf71f0085a3f8aebd889a8