Add helper cstring_to_double() and use it in PostgreSQL backend.

Replace PostgreSQL-specific and incorrect string_to_double() function with a
common (i.e. reusable) cstring_to_double() which always interprets the string
as being in "C" locale as this is what the actual strings, retrieved from the
database, use.
Merge pull request #238 from vadz/cstrtod

Add helper cstring_to_double() and use it in PostgreSQL backend.