MINOR: ssl/cli: implement "add ssl ca-file"

MINOR: ssl/cli: implement "add ssl ca-file"

In ticket #1805 an user is impacted by the limitation of size of the CLI
buffer when updating a ca-file.

This patch allows a user to append new certificates to a ca-file instead
of trying to put them all with "set ssl ca-file"

The implementation use a new function ssl_store_dup_cafile_entry() which
duplicates a cafile_entry and its X509_STORE.

ssl_store_load_ca_from_buf() was modified to take an apped parameter so
we could share the function for "set" and "add".