safewrite

Safewrite is a utility meant to safely write a file to disk.  It tries
to implement proper semantics just once on behalf of programs that
don't, and/or let them be implemented more simply.

Syntax: "safewrite targetfile command [commandargs]"

It runs the specified command as a subprocess, with its standard output
redirected into a tempfile, which will be renamed in place of the target
file if all goes well.

Checks include:
 - verify success of every system call (open, write, close)
 - write data into a temporary file
 - verify that the command exited with status 0
 - sync the file after last write
 - set tempfile permissions to match existing target's
 - use rename() to move the temporary into the official place
 - don't damage an existing target file if any problems occurred
 - try to remove the tempfile if anything bad happens

TODO
 - trap signals: unlink tempfile and die (would signal subprocess,
 but it should get SIGPIPE when we go away)
 - default file permissions if target didn't exist
 - fix makefile to install more generically on other Unix flavours;
 right now it's broken mid-transition away from RH.

adb