SSL and USER_CERT_FILE

I am using PostgreSQL's SSL support and the conventions for the key and
certifications don't make sense from the client perspective. Especially
under Windows.

I am proposing a few simple changes:

Adding two API
void PQsetSSLUserCertFileName(char *filename)
{   user_crt_filename = strdup(filename);
}
PQsetSSLUserKeyFileName(char *filename)
{   user_key_filename = strdup(filename);
}

Adding two static vars in fe-secure.c

char *user_key_filename=NULL;
char *user_crt_filename=NULL;

In client_cert_cb(...)

Add:
if(user_crt_filename)   strncpy(fnbuf, sizeof(fnbuf), user_crt_filename);
else   snprintf(fnbuf, sizeof(fnbuf), "%s/%s", homedir, USER_CERT_FILE);

and:

if(user_key_filename)   strncpy(fnbuf, sizeof(fnbuf), user_key_filename);
else   snprintf(fnbuf, sizeof(fnbuf), "%s/%s", homedir, USER_KEY_FILE);


The purpose of these changes is to make it easier to configure SSL in an
application which uses libpq.

Any comments?