user: move common-user includes to a subdirectory of {bsd,linux}-user/

user: move common-user includes to a subdirectory of {bsd,linux}-user/

Avoid polluting the compilation of common-user/ with local include files;
making an include file available to common-user/ should be a deliberate
decision in order to keep a clear interface that can be used by both
bsd-user/ and linux-user/.

Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>