Allow mmap-ing s data

Adds a new configure option: --enable-mmap
and, if mmap is enabled, two new command line options: -bsavems, -bloadms

The rationale is that when multiple ecm processes run on the same machine,
all doing ECM with batch stage 1 and the same B1, they all have their own
batch product s in memory which contains the same data for all processes.
As the batch product can become quite large with large B1, this may cause the
machine to run out of memory with many cores. By using a file-backed mmap
instead, the operating system can use shared memory between all processes.

The file format for the s save files has changed. It has a header now, with
magic number, endianness, B1 value and a checksum. The file format is
different between mmap-ed and non-mmap-ed data; the mmap-ed one uses the
host's endianness for mp_limb_t (as the disk file is just a memory image)
while the non-mmap-ed one uses mpz_out_raw() (as before) which always
writes in big-endian.