access - determine accessibility of file
int access(char *name, int mode)
Access evaluates the given file name for accessibility. If mode&4 is
nonzero, read permission is expected; if mode&2, write permission; if
mode&1, execute permission. If mode==0, the file merely need exist.
In any case all directories leading to the file must permit searches.
Zero is returned if the desired access is permitted, -1 if not.
Only access for open is checked. A file may look executable, but
exec(2) will fail unless it is in proper format.
The include file defines AEXIST=0, AEXEC=1, AWRITE=2, and AREAD=4.
Since file permissions are checked by the server and group information
is not known to the client, access must open the file to check permis‐
sions. (It calls stat(2) to check simple existence.)