bbmain/cmd: make utility functions private

[dolmen]

The main package of gobusybox binaries (see main.go) contains the following exported functions:

• AbsSymlink
• IsTargetSymlink
• ResolveUntilLastSymlink

I don't see a reason why those functions are exported. Making them private would make clearer that refactoring them or even deleting them is a way forward.

Note: This change would be helpful to go further in bbmain refactorings following #128 (which inlines IsTargetSymlink, but keeps it, unused).