Subject-Alt-Name: Correct behavior of global option --san

[TinCanTech]

--san|--subject-alt-name current behavior is currently incorrect.

Appending any value to EASYRSA_EXTRA_EXTS repeatedly inserts OpenSSL
label 'subjectAltName = ' when this label should be specified once only.

This change correctly formats EASYRSA_EXTRA_EXTS, to only begin with the
label 'subjectAltName = ' and append user values to that string.

Example Command line:

--san=DNS:server3 --san=DNS:swerveur3 --san=IP:10.2.2.2 --san=IP:10.1.1.1
--nopass build-server-full s3

Resulting certificate:

X509v3 Subject Alternative Name:
    DNS:server3, DNS:swerveur3, IP Address:10.2.2.2, IP Address:10.1.1.1

The originally required command string:

--san=DNS:server3,DNS:swerveur3,IP:10.2.2.2,IP:10.1.1.1 build-server-full s3

is also still supported.

Also, the Easy-RSA confirmation dialogue is shown as:

subject=
    commonName                = s3

X509v3 Subject Alternative Name:
    DNS:server3,DNS:swerveur3,IP:10.2.2.2,IP:10.1.1.1

Type the word 'yes' to continue, or any other input to abort.
  Confirm request details: yes

[TinCanTech]

As obvious a fix as this appears to be, I would prefer to separate SAN from other "Extra (Undefined) extensions".

Thus, EASYRSA_EXTRA_EXTS is not the preferred variable to use for SANs.

For Easy-RSA v3.2, I believe it is also time to resolve #576.
The resolution will be that support for servers named by IP address, having an automatic IP SAN added, is nonsense. Follow-up: #1091

[TinCanTech]

Superseded-by: #1093 #1096