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/shared/seccomp-util.h

@@ -23,8 +23,6 @@ extern DLSYM_PROTOTYPE(seccomp_rule_add_exact);
 extern DLSYM_PROTOTYPE(seccomp_syscall_resolve_name);
 extern DLSYM_PROTOTYPE(seccomp_syscall_resolve_num_arch);
 
-int dlopen_libseccomp(void);
-
 DECLARE_STRING_TABLE_LOOKUP_TO_STRING(seccomp_arch, uint32_t);
 int seccomp_arch_from_string(const char *n, uint32_t *ret);
 
@@ -163,12 +161,11 @@ static inline bool is_seccomp_available(void) {
         return false;
 }
 
-static inline int dlopen_libseccomp(void) {
-        return -EOPNOTSUPP;
-}
 
 #endif
 
+int dlopen_libseccomp(int log_level);
+
 /* This is a special value to be used where syscall filters otherwise expect errno numbers, will be
    replaced with real seccomp action. */
 enum {