nmsg/compat.h: new; add a compatibility header for the old "ISC" identifiers

message types with the old "ISC" vendor have been changed to use the new
"base" vendor. add a compatibility header so that code that uses the old
names continues to compile.
build system: set up a compatibility symlink for nmsg/isc/ includes

this restores compatibility for client code that includes files from the
nmsg/isc/ directory, like:

    #include <nmsg/isc/defs.h>

which should now be written as:

    #include <nmsg/base/defs.h>

but, for now, symlink nmsg/isc to nmsg/base in the include directory to
avoid breaking old code which includes headers from the "isc" directory.
build system: explicit checks and summary messages for documentation building tools

add build system checks for the two documentation building tools,
doxygen and docbook2x-man. these programs may not be available in the
build environment, so explicitly search for them and disable the
documentation targets in the makefile if they are not available. this
makes it possible to build the rest of the nmsg distribution on systems
where these tools are not available.

also print summary messages at the end of the configure run indicating
if the documentation will be built or not.
.travis.yml: don't bother building the manpage

docbook2x-man has a long dependency chain which takes a while to
download and install. don't bother building the manpage when building on
travis-ci, which should speed up the build.