picoev: handle EAGAIN or EWOULDBLOCK quietly

[4kbyte]

C recv and accept calls can reply with EAGAIN or EWOULDBLOCK on a -1 return value, which is to be expected for the asynchronous picoev event loop server.

Fix #21316 by continuing silently in these cases.

As a runtime asynchronous bug, the test is a stressful benchmark. In one terminal:

$ v run pico.v
Starting webserver on http://localhost:8089/ ...

In another (on a 24-core machine):

$ siege --concurrent=22 --reps=5000 http://localhost:8089
...
Transactions:                 110000 hits
Availability:                 100.00 %
Elapsed time:                   7.50 secs
Data transferred:               1.47 MB
Response time:                  0.00 secs
Transaction rate:           14666.67 trans/sec
Throughput:                     0.20 MB/sec
Concurrency:                   20.20
Successful transactions:      110000
Failed transactions:               0
Longest transaction:            0.96
Shortest transaction:           0.00

After this patch, there are no Error during req_read or Error during accept messages. 👍

[4kbyte]

Thank you!