Disabling warnings for certain operations

[Matheus Branco Borella <dark.ryu.550@gmail.com>]

When a call to `target_search_memory` is made, GDB will print a
warning that the memory could not be read if `simple_search_memory` is
used by the target in the current inferior. While this behavior is
perfectly sensible from the point of view of a user triggering the
call from the GDB command line directly, that same call is also
triggered by calling `gdb.Inferior.search_memory` from Python, meaning
the warning in that case will be printed even if the user has no
direct involvement with the call.

This becomes a usability problem when Python routines like Pwndbg's
search command have to scan over memory and end up hitting ranges that
`target_search_memory` fails to read, and, while the search routine
itself can gracefully handle that case, a string of warnings gets
printed, regardless. Here is an example:
```
pwndbg> search aasdqw
Searching for value: 'aasdqw'
warning: Unable to access 16000 bytes of target memory at
0x7ffff7147d05, halting search.
warning: Unable to access 16005 bytes of target memory at
0x7ffff7348000, halting search.
warning: Unable to access 16005 bytes of target memory at
0x7ffff75b9000, halting search.
warning: Unable to access 16000 bytes of target memory at
0x7ffff79a8d05, halting search.
warning: Unable to access 16005 bytes of target memory at
0x7ffff7bd0000, halting search.
```

As a workaround, I've hacked around GDB a bit, and added an option in
`target.c` (set through `set print-default-memory-search-warnings
off`) to suppress the warning message when `simple_search_memory` is
called from `default_search_memory`. Ideally, I'd like to submit some
form of achieving this as a patch, but the current solution feels
hacky and rough.

Is there a better way to do this? Or some way to query whether a range
can be read ahead of time so that calling `gdb.Inferior.search_memory`
may be skipped for those ranges, avoiding the path that would trigger
the warning altogether?