Disallow opening a POSIX message queue for execute.

O_EXEC was formerly ignored, so equivalent to O_RDONLY.

Reject O_EXEC with [EINVAL] like the invalid mode 3.
Move the PAIR32TO64() macro and the RETVAL_HI/RETVAL_LO defines to a
header file for use by other .c files.

Sponsored by:	The FreeBSD Foundation
The cap_rights_limit(2) system calls needs a wrapper for 32bit binaries
running under 64bit kernels as the 'rights' argument has to be split
into two registers or the half of the rights will disappear.

Reported by:	jilles
Sponsored by:	The FreeBSD Foundation
Regenerate after r254491.