Log Message:
* Teach the tools framework how to supply the pkgsrc version of
  makeinfo if no native makeinfo executable exists.  Honor TEXINFO_REQD
  when determining whether the native makeinfo can be used.

* Remove USE_MAKEINFO and replace it with USE_TOOLS+=makeinfo.

* Get rid of all the "split" argument deduction for makeinfo since
  the PLIST module already handles varying numbers of split info files
  correctly.

NOTE: Platforms that have "makeinfo" in the base system should check
      that the makeinfo entries of pkgsrc/mk/tools.${OPSYS}.mk are
      correct.