rfc search: Use grep -R option rather than shell globbing

[delucks]

Hello! I enjoy using this project and have recommended it to many others. When installing it on a computer with a low bash ARG_MAX, I noticed that the search functionality was breaking with "Argument list too long". To reproduce this, install rfc on a machine with a low ARG_MAX (my test machine was 262144), then:

$ rfc sync all
$ rfc search dns
/home/delucks/bin/rfc: line 116: /usr/bin/grep: Argument list too long

This PR changes rfc to use grep -R across the entire RFC cache directory rather than doing a stock grep against every element of the RFC directory using shell globbing. This should be more robust across embedded devices and other machines with a small amount of available memory. In my manual tests, all the output of the tool has been the same before & after this change. All tests pass as well:

$ ./test/tests.sh
all 5 rfc tests passed in 0.947s.

[bfontaine]

Thank you @delucks! 🎉