- 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

Add three execution tests for make(1):
- plus: execute "+command" when run with -jX -n
- ellipsis: ellipsis ("...") from variable
- empty: empty command (from variable)

Currently make(1) fails all three tests:
- plus: segmentation fault due to incorrect command list handling
- ellipsis: works in compat mode but fails in job (-jX) mode
- empty:
- compat mode: prints error message
- job mode: works but prints empty string