nmap: add sub-options for ncat, nping and ndiff

nmap: add sub-options for ncat, nping and ndiff

The forthcoming "libvirt" package will need a "nc" command with support
for Unix domain sockets in order to permit remote management using the
virt-manager utility. netcat-openbsd provides such command but requires
glibc, so we would not have remote management on uClibc-based systems.

Ncat is a much-improved reimplementation of the venerable Netcat and is
compatible with uClibc and musl. It provides features not available in
the ancient GNU netcat and its Busybox double like IPv6, proxies, and
Unix sockets.

Since 'nmap' itself is a fairly big program (~2.3 MB), we want to be
able to install only ncat. In addition, nmap requires an external
library, pcre.

So this commit adds sub-options to selectively enable/disable the
different programs part of nmap: nmap, nping, ndiff and ncat.

Finally, we symlink 'nc' to ncat if neiter netcat nor netcat-openbsd
is selected, even though ncat does not have the same interface as
netcat-openbsd.  However, since Fedora/RHEL install nmap-ncat as 'nc',
it can be assumed that packages that depend on 'nc' know how to deal
with this diversity.  For example, the virt-manager package does
that. Also user-supplied scripts can be assumed to do the right thing,
since the user also selects whether nmap-ncat, netcat or
netcat-openbsd is installed.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
[Thomas:
 - don't just handle the ncat case in a special way: create
   sub-options for all nmap tools, and use <pkg>_MAKE_OPTS and
   <pkg>_INSTALL_TARGET_OPTS to select which ones should be
   built/installed.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>