init: add support for global seccomp boot option

Setting androidboot.seccomp=global on the kernel command line shall
enable seccomp for all processes rather than just in zygote. Doing
this has a performance impact, for now it shall just be used to audit
syscall usage during testing.

Bug: 37960259
Change-Id: I6b9fc95e9bec5e2bcfe6ef0b4343a5b422e30152