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/validatefs/validatefs.c

@@ -290,7 +290,7 @@ static int validate_gpt_metadata_one(sd_device *d, const char *path, const Valid
         assert(d);
         assert(f);
 
-        r = dlopen_libblkid(LOG_ERR);
+        r = DLOPEN_LIBBLKID(LOG_ERR, SD_ELF_NOTE_DLOPEN_PRIORITY_REQUIRED);
         if (r < 0)
                 return r;