CRM_Utils_Check_Component_Env - Soften messages for read-only extDir

CRM_Utils_Check_Component_Env - Soften messages for read-only extDir

There are competing schools of thought on whether extension folders should be web-writable:

* Sometimes, the most active (or the only) admins are web-based. Making the folder writeable lets them keep extensions up-to-date through the web UI.
  This includes applying security-fixes for extensions. Thus, sites with writeable extdir are harder to attack (more secure).
* Sometimes, the most active (or the only) admins don't use the web-based admin UI, and they don't trust any web-based users to do administration.
  They don't want the folder to be writeable.
* If there's a flaw that allows writing to the filesystem, it could be escalated to writing+executing code. Thus, sites with read-only
  extdir are harder to attack (more secure).

This commit tries to accept each scenario as valid -- but communicate
better.  Instead of flatly describing the read-only dir as erroreous,
present a warning with some choice/trade-off.