the old code treated a symlink to directory as file; it tried
to calculate regular has of it. fix this by: 1) using no follow
on migration and pruning stats, and 2) the helper function to
check if it's point to directory and not calculate hash in that
case. fixes #188.

also have the output stream support writing to temporary file
and do renameat/unlinkat on close depending on if all writes
succeeded or not.

usually we are interested on the actual file's length. but
audit is interested about the link. so add a flag for this and
use it in audit.

this way we never change cwd, and relative filenames are always
parsed consistently. this also helps filename construction in many
places. this patch also changes '--root' to override location of
all configuration to be in the new root. previously it depended
on the file which one was used.

remember to increment destination pointer; and munmap the proper
base address.

and use it for /etc/init.d by default. fixes #99.

so we avoid some malloc/free calls.

apparently it needs to have both PROT_READ and PROT_WRITE. and
it needs to be MAP_SHARED for the writing to be effective. oh,
and the data needs to be preallocated with ftruncate; otherwise,
one gets SIGBUS.

avoids some copying and system calls.