Allow suppression of warnings and messages with missing restarts

[lionel-]

The attached patch makes it possible to use suppressMessages() and
suppressWarnings() with code that signals messages and warnings with
non-default protocols. For example, signalling a message with
warning() or a warning with message() are instances of non-default
protocols. In that case, the restarts that are part of the default
signalling protocols for those condition classes are missing. This
currently causes suppressMessages() and suppressWarnings() to fail
with an error. The patch fixes this by first checking that the restart
exists before trying to invoke it.

Since that is needed in multiple places in base R and in packages, I
have extracted the pattern in a dedicated function maybeInvokeRestart().
Initially I made it an argument to invokeRestart(), but it didn't
feel right the control flow semantics of a function via an argument.
Furthermore, it seems the permissive pattern should be used whenever
the condition is signalled from foreign contexts over which the
developer has no control. On the other hand, the strict variant should
be used when restarts are used for control flow effects within a
package. Given these strictly different usages, a new variant seems
more appropriate than an option.