Fix initdb for path with whitespace and at char

Hi,

On win32, initdb fails if it's path includes a space and at ('@')
character. E.g.

C:\>"C:\Program Files\user(at)company\Postgres\9(dot)3\bin\initdb(dot)exe" -D "c:\baz"
'C:\Program' is not recognized as an internal or external command,
operable program or batch file.

Here's a patch that fixes initdb by enclosing the command string in
extra double quotes being passed to popen() (similar to system() call).

Thanks,
Nikhil