tools/mkimap: report clearly missing 'configdirectory:' in imapd.conf

config_read_file() contains:

if (*p != ':') {
    snprintf(errbuf, sizeof(errbuf),
            "invalid option name on line %d of configuration file %s",
            lineno, filename);
    fatal(errbuf, EX_CONFIG);
}
*p++ = '\0';

/* remove leading whitespace */
while (*p && Uisspace(*p)) p++;

so it accepts zero spaces after the colon. Extend mkimap to accept zero
spaces after the colon in imapd.conf and to terminate fast when imapd.conf
contains no 'configdirectory:' or does not define partition.