Documented 'nopath' and 'utime_omit_ok' flags.

Exported UTIME_NOW and UTIME_OMIT under 'ALL' and 'utime' tags.

Implemented read_buf() operation, and added an implementation in
example/loopback.pl. Only tested with memory, not with FD based
mode.

Fixed fallocate() wrapper to use the correct callback number.

Altered testing infrastructure to pass logfile name to loopback.pl, and
have it open the logfile, instead of trying to open it in test/s/mount.t,
as since loopback.pl's daemonize() reopens stdout/stderr only /dev/null
normally, the logfile is worthless. This makes it functional again.
Document 'read_buf' operation.

Added accessors for the FUSE_BUF_* constants, and exported them under the
'zerocopy' and 'all' tags.

Removed unnecessary detritus from the read_buf() wrapper.