Preliminary code review for C version of initdb. Re-centralize handling

of option switches for backend, fix handling of COPY from data files so
that we won't have the newline-after-\. issue back again, add back some
comments and printouts lost from the shell script, etc.  Still needs work
for error handling; in particular the shell version worked much more
nicely for the case of a postgres executable that fails on invocation.