add mkostemp, mkstemps, and mkostemps functions and reorganize temp internals

based on patch contributed by Anthony G. Basile (blueness)

some issues remain with the filename generation algorithm and other
small bugs, but this patch has been sitting around long enough that I
feel it's best to get it committed and then work out any remaining
issues.

make mktemp match the historic behavior, and update functions that use it

the historic mktemp is supposed to blank the template string on
failure, rather than returning 0. just zero the first character so
that mkstemp and mkdtemp can still retry with O(1) space requirement.

fix major bug created from copying mkdtemp logic

major improvements to temp file name generator

use current time in nanoseconds and some potentially-random (if aslr
is enabled) pointer values for the initial tempfile name generation,
and step via a cheap linear prng on collisions. limit the number of
retry attempts to prevent denial of service attacks even if an
attacker can guess the filenames.

reformat mkstemp like mkdtemp

this is cleaner and makes it easy to impose a limit on the number of
retries later if it seems desirable to do so.

ensure standard functions mk[sd]temp don't depend on removed function mktemp

begin namespace-cleanup of standard C headers

initial check-in, version 0.5.0