Add kgetline() to kstring.c/.h

Add kgetline() to kstring.c/.h

Similar to BSD's getline() but omits the \n terminator and manages the
memory as a kstring.  Call with "(kgets_func *) fgets" to read from stdio,
or implement an fgets()-style function to read from other streams, e.g.,
a wrapper around gzgets() that reorders its parameters as per fgets().