Testing for existance of exec()

[hellopablo]

Tests for existence of exec() function so as not to raise notices on systems where "exec() has been disabled for security reasons".

[rokob]

LGTM

[rokob]

I took me some digging to confirm that function_exists actually returns false for disabled functions, because there are a lot of things online saying that it will return true but just error out. It seems that as of at least 5.3+ if a function is disabled via the ini file then this method will work. I think that is good enough.