#4400, #5462: Move random helpers to crypto-utils

#4400, #5462: Move random helpers to crypto-utils

On a way to factoring out OpenSSL support to a standalone file to ease
addition of other crypto libraries support in the future, move helpers
providing random numbers/data generation to crypto-utils.{c,h}. OpenSSL-
related functionality (generation of cryptographically strong random
data) is moved to crypto-utils-openssl.c.

Rename functions to follow currently accepted style:
* tr_cryptoRandBuf -> tr_rand_buffer
* tr_cryptoRandInt -> tr_rand_int
* tr_cryptoWeakRandInt -> tr_rand_int_weak

Fix rare case of invalid value being returned from tr_rand_int. Return
value for abs(INT_MIN) may be undefined and thus negative, and so
tr_rand_int will return negative value which is incorrect (out of
requested and expected range).