Support URI scheme for root-path in netbooting

Rather that previous attempts to add tftpfs support at the same time as NFS
support. This time decide on a proper URI parser rather than hacks.

root-path can now be define the following way:
For tftpfs:

tftp://ip/path
tftp:/path (this one will consider the tftp server is the same as the one where
the pxeboot file was fetched from)

For nfs:
nfs:/path
nfs://ip/path

The historical
ip:/path
/path

are kept on NFS

Reviewed by: tsoom, rgrimes
Differential Revision: https://reviews.freebsd.org/D10947