DoS with carefully crafted spec files

[kraptor]

Currently, it's possible to trick replace_macros() to never return, causing a DoS to software using this library with carefully crafted spec files.

The offending code in replace_macros():

while True:
    ret = re.sub(_macro_pattern, get_replacement_string, string)
    if ret != string:
        string = ret
        continue
    return ret

The simplest fix would be to add a limit on maximum recursive macro substitutions.