scanner: Fix for memleak due to unclosed file pointer

scanner: Fix for memleak due to unclosed file pointer

When including a file, it is opened by fopen() and therefore needs to be
closed after scanning has finished using fclose(), otherwise valgrind
will report a memleak.

This patch changes struct input_descriptor to track the opened FILE
pointer instead of the file descriptor so the pointer is available for
closing in scanner_destroy().

While at it, change erec_print() to work on the open FILE pointer so it
doesn't have to call fileno() in beforehand. And as a little bonus, use
C99 initializer of the buffer to get rid of the call to memset().

Note that it is necessary to call erec_print_list() prior to destroying
the scanner, otherwise it will start manipulating an already freed FILE
pointer (and therefore crash the program).

Signed-off-by: Phil Sutter <phil@nwl.cc>