docs/SSLCERTS: update the section about NSS database

without it, NSS can only work with its own internal formats. NSS also has a new

database format: https://wiki.mozilla.org/NSS_Shared_DB

Starting with version 7.19.7, libcurl will check for the NSS version it runs,

and automatically add the 'sql:' prefix to the certdb directory (either the

hardcoded default /etc/pki/nssdb or the directory configured with SSL_DIR

environment variable) if version 3.12.0 or later is detected. To check which

certdb format your distribution provides, examine the default

certdb location: /etc/pki/nssdb; the new certdb format can be identified by

the filenames cert9.db, key4.db, pkcs11.txt; filenames of older versions are

cert8.db, key3.db, modsec.db.

Usually these cert databases are empty, but NSS also has built-in CAs which are

provided through a shared library, libnssckbi.so; if you want to use these

built-in CAs, then create a symlink to libnssckbi.so in /etc/pki/nssdb:

ln -s /usr/lib[64]/libnssckbi.so /etc/pki/nssdb/libnssckbi.so

Starting with version 7.19.7, libcurl automatically adds the 'sql:' prefix to

the certdb directory (either the hardcoded default /etc/pki/nssdb or the

directory configured with SSL_DIR environment variable). To check which certdb

format your distribution provides, examine the default certdb location:

/etc/pki/nssdb; the new certdb format can be identified by the filenames

cert9.db, key4.db, pkcs11.txt; filenames of older versions are cert8.db,

key3.db, secmod.db.

     Peer SSL Certificate Verification with Schannel and Secure Transport

     ====================================================================