read-file: Avoid memory reallocations with regular files.

read-file: Avoid memory reallocations with regular files.
* lib/read-file.c: Include <sys/stat.h>, <stdio.h>, <stdint.h>.
(fread_file): With regular files, use the remaining length as the
initial buffer size.  Check against overflow.
* modules/read-file (Depends-on): Add ftello, malloc-posix, stdint,
sys_stat.