Do not obliterate errno value in the main thread during ptrace(2) call on x86.

Since ptrace(2) syscall can return -1 for non-error situations, libc
wrappers set errno to 0 before performing the syscall, as the service
to the caller. On both i386 and amd64, the errno symbol was directly
referenced, which only works correctly in single-threaded process.

Change assembler wrappers for ptrace(2) to get current thread errno
location by calling error(). Allow error interposing, as
currently allowed in cerror().

Sponsored by: The FreeBSD Foundation
MFC after: 1 week