Support openssl pem file for mac os homebrew

We discovered that mkcert did not work as expected with PHP on Mac OS when installed via Homebrew. The reason seems to be that PHP via homebrew is compiled against homebrews curl version, which in turn is using homebrews openssl, which provides his own root storage at `/usr/local/etc/openssl/cert.pem`.

This file is not updated by `mkcert -install`. Therefore requests via PHP curl will not work with mkcert certificates.

Our propose would be to detect this file and print either a warning that this is not updated, but needs to be adjusted by the user, or to update this file. In any case a warning might be useful, as I expect the file to be managed by homebrew and the package. Therefore updates might remove the added mkcert root certificate. Maybe there is a concept like hooks inside of homebrew, allowing mkcert to re add his root certificate on every update.

Used versions:
PHP Version 7.0.33
brew info openssl
openssl: stable 1.0.2s (bottled) [keg-only]
brew info curl
curl: stable 7.66.0 (bottled), HEAD [keg-only]
brew info mkcert
mkcert: stable 1.4.0 (bottled)

Workaround right now:
```
cat "$(mkcert -CAROOT)/rootCA.pem" >> /usr/local/etc/openssl/cert.pem
```