Made `flymake-phpcs-standard' more safe.

`flymake-phpcs-standard' is only safe as a local variable if the
specified standard is not pointing to a file/path to a standard.

So the safe-local-variable property is adjusted to only be safe when
the value is string-or-null-p AND it is not file-exists-p.

This might fail in the case where you specify i.e. PEAR as standard and
also have a file or folder named PEAR relative to the buffer. PHPCS will
choose the installed standard but `flymake-phpcs-standard' will consider
the setting unsafe. At least it only fails to the safe side - it won't
allow unsafe values but can mark a few safe values as unsafe.

Another approach would be to only accept all installed standards
(phpcs -i) as safe values but we'll spare the overhead of running
external commands.