implement "usermrf" fstab flag to force rm -rf * deletion on specific partitions

Change-Id: I58f4a8cedf31e6b6efd88f96a43ed525603ae311
Initialize TWPartition::User_Rm_Rf to false in constructor

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
Merge branch 'twrp2.5' into twrp2.5-safestrap