Print all mount(2) flags in ddb's "show all mounts"

Print all mount(2) flags in ddb's "show all mounts"

Quite a few flags were never accounted for in MNT_BITS, resulting in bogus
debug output such as "flags 4205800<WXALLOWED,LOCAL,ROOTFS>" (integer value
does not match string interpretation, i.e. "SOFTDEP,SWAPPABLE" is missing).

Spotted while instrumenting "show all mounts" to report a softdep panic.

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