Replace random() with pg_prng random function.

Replace random() with pg_prng random function.

Previously we used random() for choosing load balancing node. However
PostgreSQL has better random number generator: pg_prng.c. This commit
imports the file and use pg_prng_double() to generate random number in
range [0.0, 1.0).

Other notes regarding the port:

- pg_prng needs to be initialized using pg_prng_strong_seed() per
  process. Currently the only caller is child.c (per session
  process). If other process needs to use pg_prng, it needs the same
  initialization as child.c.

- Some of functions in the file were not ported because they require
  additional library: pg_bitutils.c. In the future we may revisit and
  import pg_bitutils.c.

- likely/unlikely are ignored. In the future we may revisit import
  them.

- All conditional compiling regarding "sun" or "_sun" are removed. It
  seems the platform is not used for running pgpool anymore.

- Since srandom() is not necessary any more, related code are removed
  from pgpool_main.c, child.c and pcp_worker.c.

Discussion: [pgpool-hackers: 4588] Shuffle random functions and use better random numbers
https://www.pgpool.net/pipermail/pgpool-hackers/2025-May/004589.html