scripts/namespace.pl: improve to get more correct results

Exclude more symbols from arch/x86/vdso/ and arch/x86/boot/; add some more
linker-defined symbols into exception list; add other cond_syscalls
besides "sys_*".

Signed-off-by: Amerigo Wang <amwang@redhat.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>