geom_label: Make glabel labels more trivial by separating the tasting
routines out.

While there, also simplify the creation of label paths a little bit
by requiring the / suffix for label directory prefixes (ld_dir renamed
to ld_dirprefix to indicate the change) and stop defining macros for
these when they are only used once.

Reviewed by: cem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25597