TCOVDIR and SUN_PROFDATA_DIR

The previous version of tcov (enabled by compiling with the -xa or -a flag) used an environment variable called TCOVDIR. TCOVDIR specified the directory where the tcov counter files go to instead of next to the source files. To retain compatibility with this environment variable, the new SUN_PROFDATA_DIR environment variable behaves like the TCOVDIR environment variable. If both variables are set, a warning is output and SUN_PROFDATA_DIR takes precedence over TCOVDIR.