OpenVPN: Fix tls-verify with email address in X509 identifier

OpenVPN’s TLS verification will fail when the client’s certificate
contains an “emailAddress” attribute.
The reason is that when a client certificate is uploaded to IPFire
the resulting entry in /var/ipfire/ovpn/ovpnconfig will contain
the CN and the emailAddress separated by a slash character as
certificate name.
The script used for tls-verify, however, treats everything after
the “CN=“ string in the X509 identifier as certificate name. So
if the client’s certificate happens to contain an email address
the certificate name that the TLS verification script expects will
never match the entries in ovpnconfig.

Example
The certificate “C=XX, L=Xxxxxx, O=xxx, OU=XX, CN=ovpnClient,
emailAddress=ovpnClient@example.com” will generate this name in
ovpnconfig when uploaded: “ovpnClient/emailAddress=ovpnClient@example.com”.
On the other hand, the TLS verification script expects the
certificate name “ovpnClient, emailAddress=ovpnClient@example.com”
(there’s a comma and a space and not a slash) to be in ovpnconfig
when the client tries to connect. The expected name will never match
the entries in ovpnconfig which results in an error during the
TLS handshake:

WARNING: Failed running command (--tls-verify script): external program exited with error status: 1
VERIFY SCRIPT ERROR: depth=0, C=XX, L=Xxxxxx, O=xxx, OU=XX, CN=ovpnClient, emailAddress=ovpnClient@example.com
TLS_ERROR: BIO read tls_read_plaintext error: error:140890B2:SSL routines:SSL3_GET_CLIENT_CERTIFICATE:no certificate returned

To fix this issue, all commas in the expected certificate name need
to be replaced by a slash character in order to get the certificate
name in line with the entries in ovpnconfig.