sexp: Allow white space anywhere in a hex format.

* src/sexp.c (hextobyte): Remove.
(hextonibble): New.
(vsexp_sscan): Skip whtespace between hex nibbles.

Before that patch a string

"(a #123"
"    456#")

was not correctly parsed because white space was only allowed between
two hex digits but not in between nibbles.

• Signed-off-by: Werner Koch <wk@gnupg.org>