include cleanups: remove unused headers and add feature test macros

fix (deprecated) mktemp logic and update it to match other temp functions

the access function cannot be used to check for existence, because it
operates using real uid/gid rather than effective to determine
accessibility; this matters for the non-final path components.
instead, use stat. failure of stat is success if only the final
component is missing (ENOENT) and otherwise is failure.

remove (no longer useful) namespace-protected __mktemp symbol

use memcmp instead of str[n]cmp for temp function XXXXXX checking

remove leftover unused variable in mktemp after refactoring

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.

remove ugly prng from mk*temp and just re-poll time on retry

eliminate mk*temp dependency on snprintf

this helps some tiny programs be even more tiny, and barly increases
code size even if both are used.

another return value fix for mktemp...

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.

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.

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

initial check-in, version 0.5.0