winpr/pipe: fix variable type - unsigned to signed

fcntl returns int. Comparison to 0 < always returned false.
remove re-declaration of extended pipe functions

remove them from winpr/pipe.h since they are defined and declared
in winpr/file.h
sample client: fix ReceiveChannelData prototype
sample server: fix printf format compiler warning
winpr: change GetKnownSubPath to use a const path
winpr-env: update Get/SetEnvironmentVariableA

* set last error to ERROR_ENVVAR_NOT_FOUND in GetEnvironmentVariableA
  if not found
* remove unused variables in SetEnvironmentVariableA
* add more test cases
transport: fix variable initialization

Fixes possible problem when NLA is enabled and header verification fails.
core/fastpath: fix compiler warning
Merge pull request #1798 from bmiklautz/fix/misc

Cleanup and fix compiler warnings