The header is included to get defintion for htons. In termux the
definition is in sys/endian.h, but on other systems it is in
arpa/inet.h.

arpa/inet.h includes sys/endian.h though, so including that header
works in termux as well.

Change to arpa/inet.h so that termux-api-package can be easily
compiled both in termux and for other systems.

We exit right after so this is not exactly strictly necessary, but
that might change at some point and is good practise in any case.

malloc might fail to allocate memory for child_argv. Ensure it was
successfully allocated before trying to set its elements.

And make malloc&memcpy handling clearer.

Before we potentially had an element with uninitialised memory, see
discussion in #203.

Fixes: 1ec1334 ("Initial push")