Add a way to temporarily set a gdb parameter from Python

It's sometimes useful to temporarily set some gdb parameter from
Python.  Now that the 'endian' crash is fixed, and now that the
current language is no longer captured by the Python layer, it seems
reasonable to add a helper function for this situation.

This adds a new gdb.with_parameter function.  This creates a context
manager which temporarily sets some parameter to a specified value.
The old value is restored when the context is exited.  This is most
useful with the Python "with" statement:

   with gdb.with_parameter('language', 'ada'):
      ... do Ada stuff

This also adds a simple function to set a parameter,
gdb.set_parameter, as suggested by Andrew.

This is PR python/10790.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=10790