xattr-util: merge various getxattr()/listxattr() helpers into getxattr_at_malloc() + listxattr_at_malloc()

Unfortunately fgetxattr() and flistxattr() don't work via O_PATH fds.
Let's thus add fallbacks to go via /proc/self/fd/ in these cases.

Also, let's merge all the various flavours we have here into singular
implementations that can do everything we need:

1. malloc() loop handling
2. by fd, by path, or combination (i.e. a proper openat() like API)
3. work on O_PATH

src/basic/xattr-util.h

@@ -7,26 +7,32 @@
 
 #include "time-util.h"
 
-int getxattr_malloc(const char *path, const char *name, char **value, bool allow_symlink);
-int fgetxattr_malloc(int fd, const char *name, char **value);
-
-int fgetxattrat_fake(
-                int dirfd,
-                const char *filename,
-                const char *attribute,
-                void *value, size_t size,
-                int flags,
-                size_t *ret_size);
-int fgetxattrat_fake_malloc(
-                int dirfd,
-                const char *filename,
-                const char *attribute,
-                int flags,
-                char **value);
+int getxattr_at_malloc(int fd, const char *path, const char *name, int flags, char **ret);
+static inline int getxattr_malloc(const char *path, const char *name, char **ret) {
+        return getxattr_at_malloc(AT_FDCWD, path, name, AT_SYMLINK_FOLLOW, ret);
+}
+static inline int lgetxattr_malloc(const char *path, const char *name, char **ret) {
+        return getxattr_at_malloc(AT_FDCWD, path, name, 0, ret);
+}
+static inline int fgetxattr_malloc(int fd, const char *name, char **ret) {
+        return getxattr_at_malloc(fd, NULL, name, AT_EMPTY_PATH, ret);
+}
 
 int fd_setcrtime(int fd, usec_t usec);
 
-int fd_getcrtime(int fd, usec_t *usec);
-int fd_getcrtime_at(int dirfd, const char *name, usec_t *usec, int flags);
+int fd_getcrtime_at(int fd, const char *name, int flags, usec_t *ret);
+static inline int fd_getcrtime(int fd, usec_t *ret) {
+        return fd_getcrtime_at(fd, NULL, 0, ret);
+}
 
-int flistxattr_malloc(int fd, char **ret);
+
+int listxattr_at_malloc(int fd, const char *path, int flags, char **ret);
+static inline int listxattr_malloc(const char *path, char **ret) {
+        return listxattr_at_malloc(AT_FDCWD, path, AT_SYMLINK_FOLLOW, ret);
+}
+static inline int llistxattr_malloc(const char *path, char **ret) {
+        return listxattr_at_malloc(AT_FDCWD, path, 0, ret);
+}
+static inline int flistxattr_malloc(int fd, char **ret) {
+        return listxattr_at_malloc(fd, NULL, AT_EMPTY_PATH, ret);
+}