Fix pg_enc not working if both -p and -P are provided.

Fix pg_enc not working if both -p and -P are provided.

In the original report (https://github.com/pgpool/pgpool2/issues/129:

[root@test1 bin]# ./pg_enc -p -P
db password:
encryption key:
encryption key not provided

Also I found a misleading comment:
	case 'p':			/* prompt for postgres password */

This is not appropriate since the password is not provided to
PostgreSQL. It is totally unrelated to the issue but I replaced it
with following in this opportunity:

			case 'p':			/* prompt for password to be encrypted */

Author: jinyang liu <liujy@highgo.com>
Discussion: https://www.postgresql.org/message-id/7f18c30b.237.1997555ca11.Coremail.liujy%40highgo.com
Backpatch-through: v4.2