Allow setextattr(8) to take attribute values from stdin

Add the -i option to setextattr. This option allow extended attribute data
to be provided via stdin. Add a -qq option to getextattr, which omits the
trailing newline. Together these options can be used to work with extended
attributes whose values are large and/or binary.

usr.sbin/extattr/Makefile:
Link against libsbuf which is used for processing stdin data.

usr.sbin/extattr/rmextattr.8:
Document setextattr's -i option, getextattr's -qq option, and remove
the BUG about setextattr only being useful for strings.

usr.sbin/extattr/rmextattr.c:
For setextattr operations, buffer attribute data in an sbuf. If -i
is specified, pull the data from stdin, otherwise from the
appropriate argurment.

Update usage text and argument validation code for setextattr's -i
option.

usr.sbin/extattr/tests/extattr_test.sh
Add tests for -q and -i.

Reviewed by: wblock (manpage)
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6090