MDEV-34340 netbsd fix crash on MariaDB-backup

[grooverdan]

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• [x] The Jira issue number for this PR is: MDEV-34340

Description

Without a get_exe implementation there was a null pointer exception my_realpath.

Refactor to mysys library as requested by @vuvova.

Release Notes

Fix NetBSD compatibility in MariaDB backup thanks to Thomas Klausner

How can this PR be tested?

Confirmed by contributor. (or we setup a NetBSD test)

Basing the PR against the correct MariaDB version

• [ ] This is a new feature or a refactoring, and the PR is based against the main branch.
• [X] This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• [X] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [X] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.