i386: move signal delivery code to exec_machdep.c

also move ptrace-related helpers to ptrace_machdep.c
Apply some style. Use ANSI C function definitions.
Remove MPSAFE annotations.

Reviewed by: emaste, imp
Discussed with: jrtc27
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32310