crypto/rand: rename drbg_rand.c to drbg_ctr.c

The generic part of the FIPS DRBG was implemented in fips_drbg_lib.c and the
algorithm specific parts in fips_drbg_<alg>.c for <alg> in {ctr, hash, hmac}.
Additionally, there was the module fips_drbg_rand.c which contained 'gluing'
code between the RAND_METHOD api and the FIPS DRBG.

When the FIPS code was ported to master in #4019, for some reason the ctr-drbg
implementation from fips_drbg_ctr.c ended up in drbg_rand.c instead of drbg_ctr.c.

This commit renames the module drbg_rand.c back to drbg_ctr.c, thereby restoring
a simple relationship between the original fips modules and the drbg modules
in master:

 fips_drbg_lib.c    =>  drbg_lib.c    /* generic part of implementation */
 fips_drbg_<alg>.c  =>  drbg_<alg>.c  /* algorithm specific implementations */
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4998)

crypto/rand/build.info

 LIBS=../../libcrypto
 SOURCE[../../libcrypto]=\
         randfile.c rand_lib.c rand_err.c rand_egd.c \
-        rand_win.c rand_unix.c rand_vms.c drbg_lib.c drbg_rand.c
+        rand_win.c rand_unix.c rand_vms.c drbg_lib.c drbg_ctr.c