sysrng-linux: re-open /dev/urandom every time

Prompted by the following comment by @dueno:

I also wonder why we keep the fd open for such a long time in the first place. Both OpenSSL and NSS have a similar fallback to /dev/urandom if getrandom is not available, but opens the device in one-shot, when reseeding is needed (and that's pretty rare).

I thought that could be easier to switch to such one-shot opening than to resurrect and fix tests/init_fds.c.

I'm not exactly sure about all the benefits and drawbacks of going that way though, so, please, treat that as a suggestion, not a request.

Fixes: #1188 (closed)

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code