Prefer other types to int in buffer.c

Prefer other types to int in buffer.c

This increases the volume number maximum from 2**31 - 1	to 2**63 - 1.
* src/buffer.c (record_index, inhibit_map, new_volume):
Prefer bool to int for booleans.
* src/buffer.c (volno, global_volno):
* src/system.c (sys_exec_info_script):
Prefer intmax_t to int.
* src/buffer.c (increase_volume_number): Omit by-hand check for
overflow that relied on undefined behavior.
(new_volume): Check for that overflow here instead, without
relying on undefined behavior.
(print_stats): Avoid undefined behavior if printf sums overflow,
and reliably treat printf error like overflow.
* src/common.h (add_printf): New inline function.