fix fd leak in tmpfile when the fdopen operation fails

this condition could only happen due to malloc failure.

the fdopen operation is also moved to take place after the unlink to
minimize the window during which a link to the file exists in the
directory table.

support linux kernel apis (new archs) with old syscalls removed

such archs are expected to omit definitions of the SYS_* macros for
syscalls their kernels lack from arch/$ARCH/bits/syscall.h. the
preprocessor is then able to select the an appropriate implementation
for affected functions. two basic strategies are used on a
case-by-case basis:

where the old syscalls correspond to deprecated library-level
functions, the deprecated functions have been converted to wrappers
for the modern function, and the modern function has fallback code
(omitted at the preprocessor level on new archs) to make use of the
old syscalls if the new syscall fails with ENOSYS. this also improves
functionality on older kernels and eliminates the incentive to program
with deprecated library-level functions for the sake of compatibility
with older kernels.

in other situations where the old syscalls correspond to library-level
functions which are not deprecated but merely lack some new features,
such as the *at functions, the old syscalls are still used on archs
which support them. this may change at some point in the future if or
when fallback code is added to the new functions to make them usable
(possibly with reduced functionality) on old kernels.

overhaul tmpfile, tmpnam, and tempnam functions

these all now use the shared __randname function internally, rather
than duplicating logic for producing a random name. incorrect usage of
the access syscall (which works with real uid/gid, not effective) has
been removed, along with unnecessary heavy dependencies like snprintf.

support kernels with no SYS_open syscall, only SYS_openat

open is handled specially because it is used from so many places, in
so many variants (2 or 3 arguments, setting errno or not, and
cancellable or not). trying to do it as a function would not only
increase bloat, but would also risk subtle breakage.

this is the first step towards supporting "new" archs where linux
lacks "old" syscalls.

debloat: use __syscall instead of syscall where possible

don't waste time (and significant code size due to function call
overhead!) setting errno when the result of a syscall does not matter
or when it can't fail.

make tmpfile slightly more efficient (use unlink syscall instead of remove)

make tmpfile fail after exceeding max tries.

global cleanup to use the new syscall interface

initial check-in, version 0.5.0