Extend and back-off [f]nopen() file open retries

Similar to what I do with user.tab record locks 2 months ago, this will
extend the total shared/network file open attempt duration from about 5
seconds to about 45 seconds, but with incrementing back-off.

Trying to sbbs-Linux to run from a Samba share without file open/locking
errors.

Re-synced the sbbs_t version of nopen with the nopen.c version (wasn't
special-handling EDEADLOCK failures).

Mostly we're using fnopen() these days anyway, which doesn't have an sbbs_t
version (for logging warning messages about collisions), perhaps it should.