proc.5: Describe ambiguities in /proc/<pid>/maps

proc.5: Describe ambiguities in /proc/<pid>/maps

Pathname escaping is not done properly in /proc/<pid>/maps;
because of this, different pathnames may appear the same
(verified by experiment and reading the source code).

Further details from Elvira about the relevant location in
the kernel code:

    show_map_vma() from fs/proc/task_mmu.c uses seq_file_path()
    from fs/seq_file.c to print the dentry name, which in turn
    calls seq_path() from the same file.  seq_path() uses
    d_path() from fs/d_path.c to get the path name; this is
    where the " (deleted)" part comes from. This is followed by
    mangling the string with mangle_path() (fs/seq_file.c); this
    function only replaces those characters that were supplied
    in the "esc" argument and does not bother with escaping
    anything else ('\\', for example).  The value of this
    argument comes without modifications from the initial call
    of seq_file_path() by show_map_vma(), and that is "\n".

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>