dlopen: take log_level argument and log in fallback stubs

Every dlopen_xxx() helper now takes an int log_level argument. It is
passed through to dlopen_many_sym_or_warn() (which in turn propagates it
to dlopen_verbose() for the library-not-installed case), and is used by
the fallback stub when support for the library is not compiled in to
emit a "<lib> support is not compiled in." message at the caller's level.
Callers pass LOG_DEBUG when gracefully degrading, or a higher level when
the failure should surface, and no longer need to log redundantly at the
call site.

As part of this, dlopen_bpf_full() (which already took a log_level) is
merged into dlopen_bpf() rather than keeping both.

The static inline fallbacks used to live in the headers, which required
pulling log.h in from every header that declared a dlopen_xxx(). Move
them into the .c files instead: the declaration is always outside the
#if HAVE_XXX block, the impl sits outside the outer #if HAVE_XXX wrap
with its own internal #if HAVE_XXX/#else/#endif, and apparmor-util.c,
idn-util.c, libmount-util.c and pam-util.c are now always compiled so
they can host their stubs.

src/core/execute.c

@@ -1491,7 +1491,7 @@ void exec_context_dump(const ExecContext *c, FILE* f, const char *prefix) {
                         fputc('~', f);
 
 #if HAVE_SECCOMP
-                if (dlopen_libseccomp() >= 0) {
+                if (dlopen_libseccomp(LOG_DEBUG) >= 0) {
                         void *id, *val;
                         bool first = true;
                         HASHMAP_FOREACH_KEY(val, id, c->syscall_filter) {
@@ -1910,7 +1910,7 @@ char** exec_context_get_syscall_filter(const ExecContext *c) {
         assert(c);
 
 #if HAVE_SECCOMP
-        if (dlopen_libseccomp() < 0)
+        if (dlopen_libseccomp(LOG_DEBUG) < 0)
                 return strv_new(NULL);
 
         void *id, *val;
@@ -1979,7 +1979,7 @@ char** exec_context_get_syscall_log(const ExecContext *c) {
         assert(c);
 
 #if HAVE_SECCOMP
-        if (dlopen_libseccomp() < 0)
+        if (dlopen_libseccomp(LOG_DEBUG) < 0)
                 return strv_new(NULL);
 
         void *id, *val;