The test was always useless, but, as Coverity apparently didn't realize
that isdigit() returns true only for '0'-'9', it didn't notice until we
used ND_ASCII_ISDIGIT() instead.
Addresses Coverity CID 1453475.
Some of them are locale-dependent, and all of them run the risk of
failing if you hand them a char with the 8th bit set.
Move our replacements to a new netdissect-ctype.h file, and, for the
ones that check for particular character types, add _ASCII to the name,
to indicate that only ASCII characters pass the check. Do the same for
the ones that map between cases, to indicate that they only map ASCII
letters.
For isspace(), explicitly check for the characters we care about, to
make it clearer what we're doing.
This can prevent bizarre failures if, for example, you've done a
configuration in the top-level source directory, leaving behind one
config.h file, and then do an out-of-tree build in another directory,
with different configuration options. This way, we always pick up the
same config.h, in the build directory.
Have our own routines to convert between IPv4/IPv6 addresses and
strings; that helps if, for example, we want to build binary versions of
tcpdump for Windows that can run both on NT 5 (W2K/WXP), which doesn't
have inet_ntop() or inet_pton(), and NT 6 (Vista/7/8/10), which do. It
also means that we don't require IPv6 library support on UN*X to print
addresses (if somebody wants to build tcpdump for older UN*Xes lacking
IPv6 support in the system library or in add-on libraries).
Get rid of files in the missing directory that we don't need, and
various no-longer-necessary autoconf tests.
