lib/randutils: don't break on EAGAIN, use usleep()

lib/randutils: don't break on EAGAIN, use usleep()

The current code uses lose_counter to make more attempts to read
random numbers. It seems better to wait a moment between attempts to
avoid busy loop (we do the same in all-io.h).

The worst case is 1 second delay for all random_get_bytes() on systems
with uninitialized entropy pool -- for example you call sfdisk (MBR Id
or GPT UUIDs) on very first boot, etc. In this case it will use libc
rand() as a fallback solution.

Note that we do not use random numbers for security sensitive things
like keys or so. It's used for random based UUIDs etc.

Addresses: https://github.com/karelzak/util-linux/pull/603
Signed-off-by: Karel Zak <kzak@redhat.com>