use of environment variables in connection/data string

[mko]

Currently, one use variable substitutions to make a mapfile behaving dynamically. If you do not want a user to adapt mapfile parameters but your program logic (e.g. a prepended wrapper), substitutions shouldn’t be used as a user could override it.

Environment variables could be used to e.g. use the username of an authenticated user in the data statement or to hold passwords for databases if you want to share a mapfile across different hosts with different passwords.

[mko]

Same placeholders as crypted strings, e.g. {REMOTE_USER} in order to avoid multiple search runs; may be confusing for users if they have a variable name in hex sytle like {EFEF} which would be treated as a crypted string.