Wait a maximum of 300 seconds for network send/recv in libsa

The reason for this change is that currently, a send/recv
takes many hours to time out.
This is suboptimal in the bootloader because it means for example
that NFS will take hours to fail before allowing subsequent access
methods such as gzip to be tried.

Setting MAXWAIT to 300 seconds (5 minutes) still allows slow
connections of 1Mb to be used to download a 30MB kernel file.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D18544