uucore::fsext: Fix mountinfo parsing w/ multiple optional fields
uucore::fsext: Fix mountinfo parsing w/ multiple optional fields

proc(5) mentions the following for the fields section and hyphen:

> (7)  optional fields: zero or more fields of the form "tag[:value]";
> see below.
> (8)  separator: the end of the optional fields is marked by a single
> hyphen.

IOW, there may actually be multiple optional fields, not just one, in
which case the previously hardcoded indexes for the filesystem type and
device name are now incorrect. Now, the filesystem type and device name
are parsed relative to the hypen's location, ensuring that they will be
correct regardless of the number of optional fields.

Signed-off-by: Ryan Gonzalez <75a79b57eb2e5b00b71ab48137fb286a6c004be3@collabora.com>

uutils/coreutils,uutils/coreutils,uutils/coreutils