kern/misc: Make grub_vsnprintf() C99/POSIX conformant

kern/misc: Make grub_vsnprintf() C99/POSIX conformant

To comply with C99 and POSIX standards, snprintf() should return the
number of bytes that would be written to the string (excluding the
terminating NUL byte) if the buffer size was big enough. Before this
change, the return value was the minimum of the standard return and the
length of the buffer. Rarely is the return value of grub_snprintf() or
grub_vsnprintf() used with current code, and the few places where it is
used do not need to be changed.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>