ICU: do not convert locale 'C' to 'en-US-u-va-posix'.
The conversion was intended to be for convenience, but it's more
likely to be confusing than useful.
The user can still directly specify 'en-US-u-va-posix' if desired.
Discussion: https://postgr.es/m/
f83f089ee1e9acd5dbbbf3353294d24e1f196e95.camel@j-davis.com
Discussion: https://postgr.es/m/
37520ec1ae9591f83132f82dbd625f3fc2d69c16.camel@j-davis.com