fix CLOCK_BOOTTIME use on kernels not having it

[diego-santacruz]

When the libc headers used to build mosquitto define CLOCK_BOOTTIME but the kernel where mosquitto runs does not implement CLOCK_BOOTTIME, all timestamps are wrong (uninitialized data), because there is no check for the success of the clock_gettime() calls.

This adds probing for the availability of CLOCK_BOOTTIME from mosquitto_lib_init(), falling back to CLOCK_MONOTONIC, and then modifies mosquitto_time() to use the selected clock. Probing at init time avoids having to do two clock_gettime() calls for every timestamp if CLOCK_BOOTTIME is not available.

It also fixes a similar problem in mosquitto_lib_init().

This fixes #3089

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[diego-santacruz]

I have amended the PR as I had mistakenly used the clock_t type instead of clockid_t (the compiler had not given any warning about this mismatch).

[ralight]

Looks good to me, thank you.