profiles: Optionally use relative file names for symlink targets.

* guix/build/union.scm (symlink-relative): New procedure.
* guix/build/profiles.scm: Re-export it.
(build-profile): Add #:symlink and pass it to 'union-build'.
* guix/profiles.scm (profile-derivation): Add #:relative-symlinks?.
Pass #:symlink to 'build-profile'.
* tests/profiles.scm ("profile-derivation relative symlinks, one entry")
("profile-derivation relative symlinks, two entries"): New tests.