new function perror_string extracted out of throw_perror_with_name.

new function perror_string extracted out of throw_perror_with_name.

The main purpose of this patch is to extract the part of
throw_perror_with_name that computes a string providing the system
error message combined with a prefix string.  This will become useful
later on to provide a routine which prints a warning using that
perror_string, rather than throwing an error.

gdb/ChangeLog:

        * utils.c (perror_string): New function, extracted out of
        throw_perror_with_name.
        (throw_perror_with_name): Rework to use perror_string.