Exec, Call: support -DFDEBUG
[RT #53132] Fix examples/method/Decompress.pm syntax error (kevin ryde)

and add usage docs
t/call.t: permit 2 more tests witgh -DFDEBUG
[RT #84292] Fix PIPE_PID/waitpid broken in Exec pipe_read since 5.17.6

Use sleep(0) instead of waitpid after close(pipe_in). Fixes sh and exec tests since 5.17.6