crypto/x509: define certDirectories per GOOS

crypto/x509: define certDirectories per GOOS

Split the list of CA certificate directory locations in root_unix.go by
GOOS (aix, *bsd, js, linux, solaris).

On solaris, also include /etc/certs/CA as documented here:
https://docs.oracle.com/cd/E37838_01/html/E61024/kmf-cacerts.html

Same as CL 2208 did for certFiles.

Change-Id: Id24822d6a674bbbbf4088ebb8fe8437edad232b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/248762
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>