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>

src/network/networkd-sysctl.c

@@ -108,7 +108,7 @@ int manager_install_sysctl_monitor(Manager *manager) {
 
         assert(manager);
 
-        r = dlopen_bpf(LOG_DEBUG);
+        r = DLOPEN_BPF(LOG_DEBUG, SD_ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED);
         if (ERRNO_IS_NEG_NOT_SUPPORTED(r))
                 return log_debug_errno(r, "sysctl monitor disabled, as BPF support is not available.");
         if (r < 0)