build: fix compiler warnings when for OpenBSD, NetBSD and Solaris

build: fix compiler warnings when for OpenBSD, NetBSD and Solaris

When building for NetBSD, the compiler was warning about
index into an array with a character when using isspace() and
similar macros.

IPPROTO_SCTP is also not defined by NetBSD, so there were
warnings about unused SCTP related variables.

OpenBSD complains about using sprintf rather than snprintf.
It's a good idea to use snprintf, anyway, to avoid buffer overruns,
though I believe in these particular cases sprintf was safe.
Nevertheless, snprintf is now used instead.

Solaris requires strings.h to find index().

Solaris complaints about a missing sentinel pointer unless
the terminating NULL in execl is cast to a "char *".