Improve mysqltest language

mysqltest has a lot of historical problems:

• ad hoc parser, weird limitations
• commands added as needed with no view over the total language structure
• historical code issues (e.g. casts that become unnecessary ~10 years ago)
  etc

A lot can be done to improve it.

Ideas

to simplify writing tests

• control structures, else in if, break and continue in while, for (or foreach) loop
• proper expression support in let, if, etc
• letenv command to set environment variables instead of having let do it.
• ${var} and ${${var}} variable expansion
• rich enough expressions to make resorting to sql unnecessary in most cases
  • in particular, see rpl_init.inc
• remove unused and redundant commands (e.g. system vs exec, query_vertical vs vertical_results ONCE)
• remove complex commands that do many sql statements under the hood, if they can be scripted, e.g. sync_slave_with_master
• scoped variables
• parameters for the source command
• remove dead code
• returning values from perl command (in some easier way than writing a new file and sourcing it)
  • e.g. stderr is stored in $perl ?
• running many test files on one server connection. They are supposed to be independent, so for performance reasons we can avoid restarting mysqltest for them

to simplify debugging

• make `echo` automatically print `#` at the start of every line (optionally, don't do it if it's explicitly specified — for backward compatibility)
• make most commands print itself to the log.
  • os and file operations, like exec/remove_file, etc, should strip MYSQL_VAR_DIR from paths.
• remove over-verbose treatment of rpl test failures (already done in 64dd3969481)