- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Use errx(3) instead of err(3) when checking if snprintf(3) succeeded.
snprintf(3) doesn't set errno in the tested cases.
- If the same argument reference (for example %1) was specified more than
once, the command didn't necessarily fit to the final command buffer. Fix
this using a dynamic sbuf buffer. Add a few regression tests for the case.

PR: bin/95079
No objections: freebsd-hackers