Unescape more characters

Only 4 characters need to be unescaped.

from getmntent(3):
> Since fields in the mtab and fstab files are separated by whitespace, octal
> escapes are used to represent the four characters space (\040), tab (\011),
> newline (\012) and backslash (\134) in those files when they occur in one
> of the four strings in a mntent structure.