add __sysv_signal abi-compat alias for the signal function

it should be noted that the "real" __sysv_signal, which we do not
implement, is semantically different from signal. references to
__sysv_signal arise in code built against glibc under certain
combinations of feature test macros, and are almost surely
unintentional since the legacy sysv signal behavior has fundamental
race conditions that cannot be worked around and which make it
impossible to use safely.

include cleanups: remove unused headers and add feature test macros

use separate sigaction buffers for old and new data

in signal() it is needed since __sigaction uses restrict in parameters
and sharing the buffer is technically an aliasing error. do the same
for the syscall, as at least qemu-user does not handle it properly.

header cleanup, conformance fixes - signals

initial check-in, version 0.5.0