`Spring.SetUnitTarget` has a variant with shifted args

Similar story as #2260, `Spring.SetUnitTarget` accepts 1 or 3 args that specify the target, but it's in the middle of the function, which shifts every the meanings for subsequent args. Consider adding dedicated functions (`SetUnitTargetGround` vs `SetUnitTargetUnit`, which also leaves room for `SetUnitTargetFeature` and even `SetUnitTargetProjectile` for antinukes?).

Alternatively it could accept a single-arg universal "thing" identifier, see #1374 (not realistic in the near future but I'm mentioning it as a practical benefit of 1374).

Start by reading the Spring.SetUnitTarget API and comparing the alternatives discussed in #2260 and #1374. The issue names no files or tests and does not settle whether dedicated target functions or a universal identifier is wanted, so completion criteria require maintainer agreement first.

Written by the indexing model from the issue text.