common/exechelp: Add general pipe function.
* common/exechelp-posix.c (gnupg_create_pipe): New function.
* common/exechelp-w32.c (INHERIT_{READ,WRITE,BOTH}): New macros.
(create_inheritable_pipe): Generalize so that both ends can be
inherited.
(do_create_pipe): Rename argument accordingly.
(gnupg_create_{in,out}bound_pipe): Use new flags.
(gnupg_create_pipe): New function.
(gnupg_spawn_process): Use new flags.
* common/exechelp-w32ce.c (gnupg_create_pipe): New stub.
* common/exechelp.h (gnupg_create_pipe): New prototype.- Signed-off-by: Justus Winter <justus@g10code.com>