fvec: add mmap based vector

[sebsura]

Backport of PR #1662 to bareos-23

Checklist for the reviewer of the PR (will be processed by the Bareos team)

Make sure you check/merge the PR using devtools/pr-tool to have some simple automated checks run and a proper changelog record added.

Source code quality (if there were changes to the original PR)

• [ ] Source code changes are understandable
• [ ] Variable and function names are meaningful
• [ ] Code comments are correct (logically and spelling)
• [ ] Required documentation changes are present and part of the PR

Backport quality

• [ ] Original PR #1662 is merged
• [ ] All functional differences to the original PR are documented above