tree-wide: convert dlopen_*() callers to DLOPEN_* wrapper macros

tree-wide: convert dlopen_*() callers to DLOPEN_* wrapper macros

Switch the executable-owned call sites from the plain dlopen_<lib>()
helpers to the new DLOPEN_<LIB>() macros, so that each binary that loads
an optional library at runtime now carries its own .note.dlopen note
instead of relying solely on libsystemd-shared. Each call site passes a
priority reflecting whether it hard-depends on the library (required) or
degrades gracefully without it (recommended).

Co-developed-by: Claude Opus 4.8 <noreply@anthropic.com>