fix sql error on bad virtualfull; detect parsing errors with strtod

[sebsura]

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This pr makes it so prev_jr an optional, so that code can check whether this particular member was initialized or not.
Previously the cleanup did not check whether prev_jr was initialised before it tried to clean it up (this happened if the
job ended in an error quickly).

It also changes the number parsing code somewhat. Now we check that everything is parsed, not just that something was parsed. In particular, with certain locales, we used to parse 1,5 as 1 and did not emit any errors since we managed to parse something.
Now we actually check if any characters are left over and fail the parsing if so.

Additionally a small enhancement was added to the parsing routines. Previously we could parse durations like
1 Year 5 Days 10 Hours correctly, but we could not parse numbers like 1 TB 500 GB, since the code did not loop until every token was consumed. This PR unified the duration/number parsing somewhat so that now even numbers can be split like above.

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[pstorz]

Looks good. I have enabled a former commented out test in test_edit.cc
and it fails. It seems that the last single 1 without modifier ist not accepted.