This patch adds release_error() and release_value() to KResult, making
it usable with TRY().
Note that release_value() returns void, since there is no value inside
a KResult.
This commit moves the KResult and KResultOr objects to Kernel/API to
signify that they may now be freely used by userspace code at points
where a syscall-related error result is to be expected. It also exposes
KResult and KResultOr to the global namespace to make it nicer to use
for userspace code.
