Added MYSQL_PS1 to environment variables section.

@tindex Environment variable, MYSQL_HISTFILE

@tindex MYSQL_HOST environment variable

@tindex Environment variable, MYSQL_HOST

@tindex MYSQL_PS1 environment variable

@tindex Environment variable, MYSQL_PS1

@tindex MYSQL_PWD environment variable

@tindex Environment variable, MYSQL_PWD

@tindex MYSQL_TCP_PORT environment variable

@item @code{LD_RUN_PATH} @tab Used to specify where your @code{libmysqlclient.so} is.

@item @code{MYSQL_DEBUG} @tab Debug-trace options when debugging.

@item @code{MYSQL_HISTFILE} @tab The path to the @code{mysql} history file.

@item @code{MYSQL_HOST} @tab Default host name used by the @code{mysql} command-line prompt.

@item @code{MYSQL_HOST} @tab Default host name used by the @code{mysql} command-line client.

@item @code{MYSQL_PS1} @tab Command prompt to use in the @code{mysql} command-line client. @xref{mysql}.

@item @code{MYSQL_PWD} @tab The default password when connecting to @code{mysqld}. Note that use of this is insecure!

@item @code{MYSQL_TCP_PORT} @tab The default TCP/IP port.

@item @code{MYSQL_UNIX_PORT} @tab The default socket; used for connections to @code{localhost}.