Write the following function:

char $*$vstrcat$($const char $*$first$,...)$;

All arguments of vstrcat are assumed to be strings, except for the last argument, which

must be a null pointer $($cast to char $*$ type$).$ The function returns a pointer to a dynamically

allocated string containing the concatenation of the arguments, vstrcat should return a

null pointer if not enough memory is available. Hint: Have vstrcat go through the argu

ments twice: once to determine the amount of memory required for the returned string and

once to copy the arguments into the string.