path-util: introduce new safe_getcwd() wrapper

It's like get_current_dir_name() but protects us from
CVE-2018-1000001-style exploits:

https://www.halfdog.net/Security/2017/LibcRealpathBufferUnderflow/

src/basic/path-util.h

@@ -41,6 +41,7 @@ bool is_path(const char *p) _pure_;
 int path_split_and_make_absolute(const char *p, char ***ret);
 bool path_is_absolute(const char *p) _pure_;
 char* path_make_absolute(const char *p, const char *prefix);
+int safe_getcwd(char **ret);
 int path_make_absolute_cwd(const char *p, char **ret);
 int path_make_relative(const char *from_dir, const char *to_path, char **_r);
 char* path_kill_slashes(char *path);