Define MIN and MAx macros in a single location

MIN and MAx are macros that a developer will universally expect
throughout the source. Yet, they were being defined in multiple places
across the source. Instead, define them in a single location in the
common wget.h header file and use them consistently everywhere.
Remove illogical assertion in ftp.c

A call to assert(1) will always fail and cause Wget to crash. If such a
situation does arise, Wget should invoke abort() and provide a useful
error message to the user prior to exiting.
Fix indentation in ftp.c (getftp)