FileRequest: style fixes

Pointed out by axeld, stippi and waddlesplash. Thanks for watching.
Remove B_PROT_* and related code

Use standard error codes instead.
This allows using error code returned by the underlying functions
directly, and makes it possible to use strerror for debugging. So, we
can also remove StatusString() from the various *Request classes.
Lua: update to bugfixed package

The previous version didn't work at all.
Update UserBuildConfig readme and sample

AddOptionalHaikuImagePackage is replaced with AddHaikuImagePackages.
Update HaikuWebKit package to version 1.2.3.

Older versions will not work anymore because of the API changes in
network kit (removal of nonstandard B_PROT_* status codes). x86 and
x86_64 packages have to be updated again.
Simplify cookie string-ification code.
HaikuDepot: Forgot to rename RatePackageWindow.cpp in DoCatalogs
Filerequest: UrlDecode() the request.

Makes things work even if the filename has some URL-disallowed
characters (spaces, utf-8, or otherwise).