FileSystem support for POSIX access method

From man access:

 int access(const char *pathname, int mode);

DESCRIPTION
access checks whether the process would be allowed to read, write or test for existence of the file (or other file system object) whose name is pathname. If pathname is a symbolic link permissions of the file referred to by this symbolic link are tested.

mode is a mask consisting of one or more of R_OK, W_OK, X_OK and F_OK.

R_OK, W_OK and X_OK request checking whether the file exists and has read, write and execute permissions, respectively. F_OK just requests checking for the existence of the file.

The tests depend on the permissions of the directories occurring in the path to the file, as given in pathname, and on the permissions of directories and files referred to by symbolic links encountered on the way.

The check is done with the processâs real uid and gid, rather than with the effective ids as is done when actually attempting an operation. This is to allow set-UID programs to
easily determine the invoking userâs authority.

Only access bits are checked, not the file type or contents. Therefore, if a directory is found to be "writable," it probably means that files can be created in the directory,
and not that the directory can be written as a file. Similarly, a DOS file may be found to be "executable," but the execve(2) call will still fail.

If the process has appropriate privileges, an implementation may indicate success for X_OK even if none of the execute file permission bits are set.

RETURN VALUE
On success (all requested permissions granted), zero is returned. On error (at least one bit in mode asked for a permission that is denied, or some other error occurred), -1 is
returned, and errno is set appropriately.