supplicant: fix setting pmf when the supplicant doesn't advertise support

wpa_supplicant only advertises pmf support since commit [1], which is
after 2.6. When using a version without that commit (for example,
plain 2.6), we would unconditionally set the global Pmf property to 1
(optional) and then skip setting the per-network property. The result
was that pmf was enabled without the possibility to disable it by
user. The correct behavior is instead to disable pmf on such versions.

[1] https://w1.fi/cgit/hostap/commit/?id=3cdb4ac074f76accf24a51d143db545afad2c90b

#129
4 jobs for bg/pmf-fix-issue129 in 27 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #139064
build_f28

00:27:50

passed #139065
build_f_latest

00:18:33

passed #139066
allowed to fail
build_f_rawhide

00:24:52

passed #139063
allowed to fail
checkpatch

00:01:07