Slightly revamp app-tools.  Provide temporary compat wrappers for old usage.

User-visible changes:

rename rumpxen-app-foo -> rumpapp-xen-foo
  rationale: easier to see what is the platform, hopefully useful if
             the tools see usage beyond rump kernels on Xen

adjust usage of make so that the actual make is passed in the env
via $RUMP_APPTOOLS_MAKE, defaults to "make"
  rationale: matches usage of make with actual make

introduce rumpapp-xen-gmake
  rationale: saves typing RUMP_APPTOOLS_MAKE=gmake
add missing "$@" for exec'ing the real configure script