compat: fix `pthread_mutex_timedlock`

[jktjkt]

compat: return ETIMEDOUT when timing out

refactor: compat: simplify waiting

This code looked a bit fishy:

• There's no need to check up front whether we're dividing a zero or non-zero number by some constant.

• The conversion to milliseconds is actually not needed at all because it gets immediately converted to back to nanoseconds.

[jktjkt]

I accidentally left this one marked as a draft; you can merge it now (and please also https://github.com/CESNET/libnetconf2/pull/424).