Invalid command line containing "%canonical_option%" causes hang when what() invoked on resulting exception. #163

Tested on program_options from boost 1.91.0, built on linux with g++ 13.3.0 on Ubuntu subsystem on Windows. Tested as well with program_options 1.81.0 on windows compiled with vc17, and found to be impacted.

During fuzzing, found that providing the option
-%canonical_option%
will cause a hang in any program that uses program_options and uses err.what() to print the invalid argument. My assumption is the field %canonical_option% is used in a string used in error reporting, and is repeatedly replaced with the user's invalid argument. If the user's invalid argument contains this magic string, the attempted replacements will continue forever.

Additionally, strings appended onto the end of this invalid argument will cause increasing memory usage, likely eventually running out of memory. I do not know the behavior in this case - if what() has additional error handling to catch a memory allocation failure, and the program will recover, or if this will result in a crash. The memory consumption appears to grow linearly - even providing multiple %canonical_option% flags, e.g. -%canonical_option%%canonical_option%%canonical_option%` seems to only cause linear growth in memory usage as the program runs. There may be additional substitutions that could be affected, but this is the only one that my fuzzing found.

Security implications: Generally, program_options is only used for processing command lines, so this only affects machines where a malicious actor can run a command line, or inject arguments that end up on a command line. Generally, the only program affected is the program launched, but there are cases where the command line may be passed to a service which then does the parsing, possibly logging any error codes. In such cases, long running services can be caused to have an infinitely looping thread, which continuously allocates memory.

Example program reproducing this hang:
`
#include "boost/program_options/errors.hpp"
#include

int main()
{
boost::program_options::unknown_option err("-%canonical_option%");
std::cout << err.what() << std::endl;
return 0;
}
`
Program hangs and nothing is printed.