Functions to quote and unquote pathnames in C-style.

Functions to quote and unquote pathnames in C-style.

Following the list discussion, define two functions, quote_c_style and
unquote_c_style, to help adopting the proposed way for quoting funny
pathname letters for GNU patch.  The rule is described in:

    http://marc.theaimsgroup.com/?l=git&m=112927316408690&w=2

Currently we do not support the leading '!', but we probably should
barf upon seeing it.  Rule B4. is interpreted to require always 3
octal digits in \XYZ notation.

Signed-off-by: Junio C Hamano <junkio@cox.net>