tftpd already requires sigaction() to compile, so there is no reason
to use anything else. It also allows for nicer combination of flags.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
bsd_signal() may not be defined, and there is really no reason to even
try to use it if sigaction() is avaiable; using sigaction() guarantees
the semantics we really want.
Replace uses of common variables with explicit instantiation and
extern declarations in a header file.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>