crypto/internal/boring: error when reading system-wide fips mode is ignored

[qmuntal]

We should take an informed decision about what to do with errors found while reading /proc/sys/crypto/fips_enabled.

I'm hesitant to panic if there is an error reading /proc/sys/crypto/fips_enabled as this code will be always executed when running Go binaries. If an app wants to be 100% sure that is running in FIPS mode it should use boring.Enabled() or GOLANG_FIPS=1.

See #291 (comment) for more context.