Provide useful HINT to user who passes -P into command line

[stuartpa]

C:\Users\user>sqlcmd -U sa -P blahblah
sqlcmd.exe: error: unknown flag -P, did you mean one of "-c", "-C", "-d", "-E", "-U", "-i", "-o", "-q", "-Q", "-S", "-X", "-G", "-x", "-v", "-a", "-l", "-H", "-K", "-N", "-b", "-V", "-m", "-F", "-r", "-h", "-u", "-?"?

Let the user know in this "sqlcmd: error:" that this version of sqlcmd requires -P to be passed in as an environment variable, which I can never remember off the top of my head, because MSSQL on Linux used SA_PASSWORD, which is now MSSQL_SA_PASSWORD, but for SQLCMD it's SQLCMDPASSWORD.