Fixed invalid Errno::EAGAIN handling in Stream#syswrite

[stopiccot]

Imaginary test:

• do_write is called with 300 bytes string
• syswrite is called with 300 bytes string
• total_written is 0
• write_nonblock is called with 300 bytes string
• write_nonblock writes only 100 of them
• written and total_written are now 100
• write_nonblock is called with 200 bytes string
• write_nonblock writes only 100 of them
• written is now 100, total_written is now 200
• write_nonblock is called with 100 bytes string
• Errno::EAGAIN is thrown
• in do_write() we are retrying sys_write again with remaining 100 bytes
• new instance of syswrite is called with 100 bytes string
• total_written is 0
• write_nonblock is called with 100 bytes string
• write_nonblock writes 100 bytes
• written and total_written are now 100
• syswrite returns result 100 instead of 300 to do_write

Looking at FIXME in syswrite i think maybe all this code should be refactored but I wanted my fix to be as small as possible. Also I don't know how to implement unittest for this bug so it's up to you.