regress: Allow harness code to wrap main if it wants to

regress: Allow harness code to wrap main if it wants to

The fuzzer is going to want to wrap main().  This is a bit awkward
because everyone defines it their own way (in ways which are obviously
equivalent but which trigger compiler warnings).  Our warnings mean we
have to make sure the comiler sees a declaration.

Luckily the only occurence of main() anywhere in one of the client
programs is (necessarily) the definition of main, which is always
defined to return int.  So we can arrange the expansion to produce
both declaration and definition of Hmain.

Then in hnonfuzz.c we need to declare Hmain.  (Which we can't declare
in the global header file.)  We hope that type errors in main are
unlikely.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>