Enhanced install_p4perl.sh:
Added '-d' flag to provide a way to pass Perl configuration
parameters on to the Perl Configure utility, e.g. -Dusethreads.
Added '-f' flag to ignore failed Perl or P4Perl test suites.
Test suites are still run, but errors are converted to warnings
with this flag.
Added display of original command line to output, and minor
cosmetic tweaks.