Add ability to set a higher maximum block size for TFTP

[nshalman]

Description

This PR adds the ability to set a higher maximum block size for TFTP transfers.

Why is this needed

We are troubleshooting some misbehaving NICs that might benefit from support for larger block sizes. They appear to request 1468 which makes sense on a link with MTU of 1500.

How Has This Been Tested?

Tested with atftp

How are existing users impacted? What migration steps/scripts do we need?

The default of 512 is preserved and made explicit in this codebase.

[nshalman]

If this proves useful it would probably benefit from some unit testing exercising the code paths which I have not done yet.

[codecov]

Codecov Report

Merging #93 (80c98a1) into main (c92a076) will increase coverage by 97.28%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           main      #93       +/-   ##
=========================================
+ Coverage      0   97.28%   +97.28%     
=========================================
  Files         0        5        +5     
  Lines         0      442      +442     
=========================================
+ Hits          0      430      +430     
- Misses        0        8        +8     
- Partials      0        4        +4     

Files	Coverage Δ
cmd.go	96.25% <100.00%> (ø)
ipxedust.go	96.02% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[nshalman]

Explicitly tested and confirmed working:

[nix-shell:~/ipxedust]$ jobs
[1]+  Running                 ./bin/ipxe-linux -tftp-addr 127.0.0.1:2069 -log-level debug -tftp-blocksize 4096 &

[nix-shell:~/ipxedust]$ rm undionly.kpxe ; echo get undionly.kpxe | atftp 127.0.0.1 2069 --option "blksize 1486" --trace 2>&1 | tail
{"level":"info","event":"get","filename":"undionly.kpxe","uri":"undionly.kpxe","client":{"IP":"127.0.0.1","Port":37290,"Zone":""},"macFromURI":"","v":0,"logger":"ipxe","bytesSent":89859,"contentSize":89859,"caller":"github.com/tinkerbell/ipxedust/itftp/itftp.go:111","time":1696524693030,"message":"file served"}
sent ACK <block: 57>
received 1. DATA <block: 58, size 1486>, update last received block: 57 → 58
sent ACK <block: 58>
received 1. DATA <block: 59, size 1486>, update last received block: 58 → 59
sent ACK <block: 59>
received 1. DATA <block: 60, size 1486>, update last received block: 59 → 60
sent ACK <block: 60>
received 1. DATA <block: 61, size 699>, update last received block: 60 → 61
sent ACK <block: 61>
tftp>