sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

linux(4): Add kernel signal code definitions

In the next commit I'll convert the native signal codes into the Linux codes,
since they are not 1:1 mapped.

MFC after: 2 weeks

linux(4): Move signal codes definitions to the appropriate header

In the Linux the struct siginfo related bits are placed into the siginfo.h
header.

MFC after: 2 weeks

linux(4): Rework the definition of struct siginfo to match Linux actual one

Rework the defintion of struct siginfo so that the array padding
struct siginfo to SI_MAX_SIZE can be placed in a union along side of the
rest of the struct siginfo members. The result is that we no longer need
the __ARCH_SI_PREAMBLE_SIZE or SI_PAD_SIZE definitions.

Move struct siginfo definition under /compat/linux to reduce MD part.
To avoid headers polution include linux_siginfo.h in the MD linux.h

MFC after: 2 weeks