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 <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,jardasgit/libvirt,crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,crobinso/libvirt,olafhering/libvirt,jardasgit/libvirt,zippy2/libvirt,jardasgit/libvirt,jfehlig/libvirt,libvirt/libvirt,libvirt/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,jardasgit/libvirt,jfehlig/libvirt,nertpinx/libvirt,jfehlig/libvirt,nertpinx/libvirt