Add sanity checks for often blocked functions

[sybrew]

Blij Online reported that PHP function set_time_limit() was unavailable.

We should add a function_exists() check for this function and any other function that might be blocked.
Since we depend on WordPress, let's see what they block. We can safely assume that whatever they don't block is free to use.

Here, I'm only listing functions I recall using before in any project:

• set_time_limit()
• session_status()
• ini_get_all()
• error_reporting()
• realpath()
• hash()
• ini_get()
• getenv()
• sys_get_temp_dir()

Do check these functions:

Looking at TSF and Extension Manager, we only use this function from that list:

• set_time_limit()

We don't use these in TSF or Extension Manager, but we should check for these when we do:

• session_status()
• ini_get_all()

Ignore these functions:

WordPress implemented checks for these inconsistently, so they might as well be removed on their end since anyone using WordPress should unblock these to avoid encountering unexpected errors:

• error_reporting()
• realpath()
• hash()
• ini_get()
• getenv()

Lastly, this function is checked consistently in WordPress but not in their dependencies (PHPMailer and Simplepie), causing an issue for most users as well:

• sys_get_temp_dir()