Improve File I/O overflow detection in gdbserver (PR server/23198)

Improve File I/O overflow detection in gdbserver (PR server/23198)

Currently, the function used by gdbserver to parse integers from
received File I/O commands will detect overflow and fail for any value
over 0xfffffff.  Among other things, this has the effect of limiting
the file offsets for reading or writing to about 268MB which can be
insufficient for particularly large libraries.

This change allows the parsing of integers up to the true maximum
positive value of 0x7fffffff, increasing the file size limit to about
2GB.

gdb/gdbserver/ChangeLog:
2018-05-23  Erik Kurzinger  <ekurzinger@nvidia.com>

	PR server/23198
	* hostio.c (require_int): Do not report overflow for integers
	between 0xfffffff and 0x7fffffff.