Move to using vsnprintf instaed of BSD based ErsVSPrintf

      Remove all the DPUBLIC, DPROTOTYPES etc. macros - presume modern C compiler.

      Add sprintftest.c, and a test target in dmakefile.  This allows us to do
	basic tests on ErsSPrintf() and ErsSVPrintf().

	Need to add more tests 	here to check rest of ERS, but do to handle not
	building with DRAMA.

      Replace BSD version of ErsVSPrintf() by the use of the modern portable
	implementation - vsnprintf().  ErsVSPrintf() and ErsSPrintf() both
	now call that routine for the formatting.  As well as removing the
	BSD license code, this will ensure we are as up to date as the OS is in
	terms of printf() formatting support.