mingw.c: Fix complier warnings for a 64 bit msvc

mingw.c: Fix complier warnings for a 64 bit msvc

Remove some complier warnings from msvc in compat/mingw.c for value
truncation from 64 bit to 32 bit integers.

Compiling compat/mingw.c under a 64 bit version of msvc produces
warnings. An "int" is 32 bit, and ssize_t or size_t should be 64 bit
long. Prepare compat/vcbuild/include/unistd.h to have a 64 bit type
_ssize_t, when _WIN64 is defined and 32 bit otherwise.

Further down in this include file, as before, ssize_t is defined as
_ssize_t, if needed.

Use size_t instead of int for all variables that hold the result of
strlen() or wcslen() (which cannot be negative).

Use ssize_t to hold the return value of read().

Signed-off-by: Sören Krecker <soekkle@freenet.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>