uml: redo host capability detection and disabling

Redo how host capabilities are recorded at startup and disabled on the
command line.

There are now explicit variables saying what's been disabled by the
command line rather than the implicitness of the have_* variable being
zero.  The capability variables now start at zero and are set to one
as their capabilities are found to be present on the host.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>