#59 is_valid_email_addr improvement

[pk910]

hi

currently srvx checks only if the entered email address has at least one @
email adresses like
xy@*
@@
etc.. are still possible

(current code)
static int is_valid_email_addr(const char *email) {
return strchr(email, '@') != NULL;
}

i've improved this function a little bit to check multiple things:
* 1st check: there need to be one @
* 2nd check: username (bevore @) must be at least 1 char long and out of a-zA-Z0-9._%+-
* 3rd check: there need to be at least 1 dot in the domain part and all characters out of a-zA-Z0-9-
* 4th check: TLD must be <= 5 chars, no special chars

[pk910]