More janitorial work

More janitorial work

Install vpp api header files in /usr/include/vpp-api, instead of
/usr/include/api. Someone will eventually complain if we continue with
the status quo.

Generate /usr/bin/vpp_plugin_configure, to correctly configure
standalone plugin compilation against header files installed from the
dev package.

If a plugin's CFLAGS don't precisely match the installed vpp engine
binary, subtle misbehavior can and will occur. Example: the ip4/ip6
main_t structure size / member offsets depend on DPDK=[0|1]. Screw
that one up, and your brand-new configurable ip feature will
mysteriously fail to appear, even though the plugin loads perfectly.

Change-Id: I20c97fe1042808a79935863209d995c31953b98c
Signed-off-by: Dave Barach <dave@barachs.net>