Extract substr into local variable

with the newer pb2 testing environments

a "if"
Bug #41913 mysqltest cannot source files from if inside while
Some commands require additional processing which only works first time
Keep content for write_file or append_file with the st_command struct
Add tests for those cases to mysqltest.test

This fixes the regression introduced in 5.1 that prevents 64 bit builds on Intel while still keeping the core2 hack operational so the cluster can build.

Remove a self assignment.
Rework a few constructs to avoid a potential overflow.

Based upon patch contributed by Michal Hrusecky

On 64-bits machines the calculation gets the wrong types and results
in very large numbers. Fixed by explicitly cast month to (int)

Failing to connect would release parts of the MYSQL struct.
We would then proceed to try again to connect without re-
initializing the struct.

We prevent the unwanted freeing of data we'll still need now.