util: fix warning of equal values on logical OR

util: fix warning of equal values on logical OR

shared/util.c: In function ‘read_str_safe’:
shared/util.c:211:24: warning: logical ‘or’ of equal expressions [-Wlogical-op]
    if (errno == EAGAIN || errno == EWOULDBLOCK ||
                        ^~
shared/util.c: In function ‘write_str_safe’:
shared/util.c:237:24: warning: logical ‘or’ of equal expressions [-Wlogical-op]
    if (errno == EAGAIN || errno == EWOULDBLOCK ||
                        ^~

This is because EAGAIN and EWOULDBLOCK have the same value. Prefer
EAGAIN, but add a static assert to catch if it's not the same in another
architecture.