find: added xattr and cap predicates

* find/defs.h (typedef PREDICATEFUNCTION): added pred_xattr and pred_cap
* find/parser.c:
(parse_xattr): added function
(parse_ixattr): added function
(parse_cap): added function
(parse_icap): added function
(insert_cap): added function
(insert_xattr): added function
(parse_table[]): added entries
* find/pred.c
(pred_cap): added function
(pred_xattr): added function
(pred_table[]): added entries for pred_cap and pred_xattr
* find/tree.c (costlookup[]): added entries for pred_cap and pred_xattr
* find/find.1:
added -xattr predicate information
added -ixattr predicate information
added -cap predicate information
added -icap predicate information
* find/Makefile.am: include libcap for -cap
* find/testsuite/excuses.txt: -xattr and -cap require root privileges
                                      to test, some planning needed