Remove a sketchy use of asprintf() passing a union by value.

The original code was passing a union by value, and doing odd things
on Solaris/SPARC (where "odd" rhymes with "SIGBUS").  Replace it with
an exploded switch/case block for all the enum values.  Also use the
string literals so that we get compiler checking of the format string
vs. the type of the actual arguments.

cmr=v1.7.4:revier=hjelmn:subject=Fix MCA base var to not pass union by value
George correctly pointed out that there's no need for this test: it
effectively exists elsewhere in the code already.