Add option to store the session encryption key.

[simo5]

With the new 'file:' sytnax a session key can be automatically generated
the first time mod_auth_gssapi runs and stored on the filesystem.

[simo5]

@frozencemetery @Frenche can you review ?
(Tested and works)

[frozencemetery]

I would prefer for the "file:" branch of the if statement (L1268-L1322) to be broken into smaller pieces (or a helper function, or something), but otherwise this looks good to me.

[iboukris]

Perhaps this sentence could be revised

[simo5]

Will do.

[iboukris]

Maybe /var/ is a better location for an auto-generated binary file

[frozencemetery]

Could go either way, I think. It's going to be up to administrators (and distro packagers, I suppose) what the path is here since there isn't a default.

[iboukris]

Ok, was just a thought.

[simo5]

I can change the example to point to /var/lib/httpd/secrets; it is a good subliminal suggestion :-)

[simo5]

Ok all concenrs should be addressed, I also changed a test to actually use this feature.
however running make test I saw one test failing (it fails also reverting this commit so it is a pre-existing issue).
@frozencemetery can you look into whether make test fails for you ?

[frozencemetery]

master is passing on fedora 24, failing the first SPNEGO test on debian testing, and trips an assert inside apache on rawhide:

[Tue Dec 20 18:02:34.459405 2016] [core:emerg] [pid 1412] (22)Invalid argument: AH00024: Couldn't set permissions on the mpm-accept mutex; check User and Group directives
(22)Invalid argument: could not create accept mutex
AH00015: Unable to open logs

I am happy with the code as written here.