Parsing `mount options=x` results in "Passed unknown options keyword to
MountRule: x", while parsing `mount options=xy` results in "Can't parse mount rule".
This difference happens because the code checks (besides the list of
known options) for a regex `([A-Za-z0-9])` which only matched a
single-character unknown option.
Change that regex to also match multiple characters, and also allow to
match `-` (used in some known mount options, so it's likely that it also
gets used in so far unknown mount options)
Known Bugs:
Will allow multiple letters in the () due to translation/unicode issues with regexing the key.
User input will probably bug out in a different locale.
9a035cb0ee