Fix mount rules encoding (respin)

This is respin of !333 (merged) series by @jjohansen

We had a private chat discussion with John and I've suggested some help from my side as this task is critical for LXD/LXC project.

Changelog:

• rebased to an actual tree
• addressed review comments from @wbumiller and @setharnold
• fixed compiler warnings about class_mount_hdr is uninitialized
• infinite loop fix
• MS_MAKE_CMDS bitmask value fixed
• fixed condition in gen_flag_rules to cover cases like mount options in (bind) /d -> /4, when flags are empty and only opt_flags are present
• marked some tests as a FAIL case behavior was changed after parser: add conflicting flags check for options= conditionals commit