fix handling of ' ', \t, \n and \ in mtab entries

From `getmntent()`'s manpage:

> 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.  The
> routines  addmntent()  and  getmntent()  will   convert   from   string
> representation to escaped representation and back.

This code add handling for these special characters.