doc/gdb.texinfo: Add EIO and ENOSYS errno values

doc/gdb.texinfo: Add EIO and ENOSYS errno values

This patch adds the EIO and ENOSYS errno values supported by GDB's
File-I/O to section E.14.9: Errno Values of the GDB manual [1] that were
presumably inadvertently omitted; both can be seen in the enum
fileio_error:gdbsupport/fileio.h and corresponding function
host_to_fileio_error:gdbsupport/fileio.cc as FILEIO_{EIO,ENOSYS}.

FILEIO_SUCCESS remains excluded from the manual, because its stated
purpose (commit b872057a6) is to internally represent the absence of an
error value from the remote, and it's not actually an error number.

[1] https://sourceware.org/gdb/current/onlinedocs/gdb.html/Errno-Values.html

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>