In some situations it is possible that a user might not want GDB to
try and access source code files, for example, the source code might
be stored on a slow to access network file system.
It is almost certainly possible that using some combination of 'set
directories' and/or 'set substitute-path' a user can trick GDB into
being unable to find the source files, but this feels like a rather
crude way to solve the problem.
In this commit a new option is add that stops GDB from opening and
reading the source files. A user can run with source code reading
disabled if this is required, then re-enable later if they decide
that they now want to view the source code.
fde1a9a3ee