vpnmain.cgi: Fix writing ESP settings for PFS ciphers
The changes introduced due to #12091 caused IPsec ESP
to be invalid if PFS ciphers were selected. Code has
to read "!$pfs" instead of just "$pfs", as it should trigger
for ciphers _without_ Perfect Forward Secrecy.
Fixes #12099
Signed-off-by: Peter Müller <peter.mueller@ipfire.org> Cc: Michael Tremer <michael.tremer@ipfire.org> Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>