meson: Drop checks for some functions and header files

meson: Drop checks for some functions and header files

In meson.build, we check for presence of unshare() function
explicitly (even though there is the function usability check a
few hundred lines below), but never have any code depending on
HAVE_UNSHARE. The same applies to stdarg.h and sys/sysctl.h
header files - either we simply include them or guard their
include using different conditionals.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>