swapon: fix swapon headers and syscalls

swapon: fix swapon headers and syscalls

This patch scrubs anything related to SWAPON_NEEDS_TWO_ARGS as this is only
relevant for < linux-1.3.2 and i say it's about time we let it go.

I also cleaned up the system call fallback logic as the _syscall#() macros are
the deprecated interface in favor of the real syscall() function.

The asm/page.h gets the boot as nothing in swapon.c utilizes defines from it
while unistd.h gets always included as defines/functions are used from it other
than just swapon().

Signed-off-by: Mike Frysinger <vapier@gentoo.org>