syscall: separate ProcSysAttr and forkAndExecInChild for FreeBSD

syscall: separate ProcSysAttr and forkAndExecInChild for FreeBSD

To allow adding fields to ProcSysAttr which are supported on FreeBSD but
not on other BSDs.

For now exec_freebsd.go is an exact copy of exec_bsd.go with adjusted
build tags and copyright year.

For #46258
For #46259

Change-Id: I7667a0cdf1ca86ef64a147b77c06db70c5f8eb90
Reviewed-on: https://go-review.googlesource.com/c/go/+/355569
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>