Revise use of non-X malloc, calloc, realloc and strdup

[jimklimov]

Similar to

• Use str* API to check for valid numeric value #677
• use str* API in place of isdigit() in server/conf.c #676
• Stop using atoi()/atol()/strtol(), convert all to str_to_*() and str_to_*_strict() #966

Use NUT provided facilities for safer and more reliable coding instead of base system routines with same error-checking copied all around the codebase.