Don't assume snprintf() returns the number of characters formatted.

On some UN*Xes (such as some versions of HP-UX), it doesn't.
Do our own isascii(), isprint(), isgraph(), and toascii().

We do *not* want the behavior of isprint() and isgraph() to be
locale-dependent - we want both of them to return "true" only for ASCII
characters.

We have to do our own isascii() and toascii() on non-UN*X systems
anyway, so let's just do all of them ourselves.
Use the new libpcap <pcap/nflog.h> for NFLOG definitions and declarations.

Use the header file for information about LINKTYPE_NFLOG files.

Don't build the NFLOG printer if we don't have that header file.

Fix some checks for pcap/XXX.h header files to look for
"tcpdump-stdinc.h" rather than <tcpdump-stdinc.h>, so that we don't fail
to find that header.