installer: Detect APC for MainCacheType in CLI installer

installer: Detect APC for MainCacheType in CLI installer

The web installer did this already, but with the CLI installer,
the generated LocalSettings.php always contained the following
hardcoded:

 $wgMainCacheType = CACHE_NONE;

Combined with the fact that in WMF CI, the generated local settings
is applied *after* the inclusion of Quibble settings and
DevelopmentSettings, meant that it was not possible to enable
object caching.

For now, make it match the behaviour of the web installer and thus
output $wgMainCacheType = CACHE_ACCELL if we detect a supported
implementation in the PHP runtime.

For later we should probably:

* Make this an option to install.php,
* or, change Quibble to append its overrides, instead of
  prepending. So that DevelopmentSettings actually after the
  generated LocalSettings.

Bug: T225496
Change-Id: I3f43cd054ce71d0f1b2395302e8ef9ee2f6b01c2